goudrons

Sounds like piston slap.
 
Mine sounded like this before I had it rebored, the skirt of the piston slapped the bore causing an identical rattle.
 
You'll need to get the bore and piston measured properly all the way down.
 
 

To get at the main bearings you need to split the cases, this can be tricky without the right tools.
 
First off, get everything you need before hand, new bearings (make sure they are C3 rated mains), gaskets, seals, tools etc, it sounds silly but leaving an engine apart longer than needed and it's likely you'll forget where everything goes!
 
Some items are best removed while the engine is still in the bike, particularly the tight nuts of the clutch centre, flywheel and perhaps primary gear, otherwise you have trouble holding the engine still while applying enough torque to the nuts when it's on a bench.
 
Store the bits you remove together, like all the clutch items in one container and so on.
 
A few sheets of cardboard with a rough online of the engine (and which side it is) drawn on will help store the casing bolts, just push them through in the correct postions they came out.
 
A tool like this will hold the clutch centre to undo the nut.
It can also be used to hold the flywheel and primary gear too, so it's a bargain!
http://www.demon-tweeks.co.uk/Product.do?method=view&n=678&p=547524&c=215&utm_source=Google&utm_medium=Base&utm_campaign=Hand%20Tools&gclid=CL2N8sDWmMUCFSMUwwodpa4AqA
 
 
The correct flywheel puller is needed.
http://www.splatshop.co.uk/sherco-flywheel-puller.html?utm_source=google&utm_medium=shopping&utm_campaign=shopping&gclid=CMHLy-7XmMUCFernwgodd2oAow
 
 
Once you're down and ready to split the cases many will use a case splitter if they have one, but with a bit of care you can tap and wiggle them apart, but be gentle and take your time.
You'll ruin the case faces (and sealing will become an issue) levering them apart, so don't do it!
 
You need to heat around the cases/bearing to remove and install them.
Once the cases around the main bearings are hot enough (and expanded) the bearings should "tap" out.
If you find you are tempted to "bash" them out, they ain't hot enough!
Plenty of posts on youtube for that though.

Yes, replace the cable guide on top of the carb.
If it's snapped off clean, you might not be able to get the threaded part out of the carb top, so you might need another of those too.
Try http://www.allensperformance.co.uk/index.html
They seem to stock most Mikuni parts, though I don't know if the bent cable guide on a Rev 3 is a standard Mikuni part, but I'm sure they'll sort you out with something suitable.
The guide it's self shouldn't really need tightening up into the carb top, just screw it all the way in with your hand, then back it out so the cable is at the correct angle (guide points forward) then the lock nut to carb top is nipped up.
The Mikuni Carb on these needs the float hieght setting spot on, search on here (or Google) there is a couple of guides on how to set these correctly.
There is no clamp between mid box and the front pipe.
Inside the mouth of the mid box are a couple of rubber O rings that fit into recesses in the mouth of the mid box, these seal up the joint.
http://www.beta-uk.com/images/stories/spares-pdfs/2004/rev3_2t_04_200-250-270cc_frame.pdf
Here's the engine one too
http://www.beta-uk.com/images/stories/spares-pdfs/2004/rev3_2t_04_200-250-270cc_engine.pdf
If it's been on and off a few times, they can get knock out of their recesses or damaged when the front pipe is pushed in.
A smear of grease should help hold them in and lude them up when refitting the front pipe.
If it's been stored, it worth checking over a few bits before you drag it miles out for a first ride.
Foam air filters can be washed out with soapy water, left to dry and a light oiling of two stroke applied (not soaked though)
Clean out the air box too, as they can get mucky water dribbling in.
Check the swinging arm bearings and bushes for play, clean and regrease them, do the same with the head stock bearings.
Check the wheels bearings for play.
Check the wheels are true (or near enough!).
Check the sprockets aren't worn and hooked over and the chain in it good nic, that's it's free from grit and sh*t. Oil it up well then apply a chain wax to hold the oil on.
Check these is a little play (a little slack) in the brake (both F and R) and clutch lever operation, around 2 mm of movement before the rods operate the master cylinder pistons should be about right (see hand book).
If you expect to fall off (like the rest of us) loosen the brake lever and clutch lever clamps to the bars ever so slightly, just enough so they will move if you drop the bike, could save broken levers and early bath.
Go over all the nuts and bolts and check their torque, I would personally redo both brake discs and rear sprocket bolts and apply thread lock to them.
Check the rear tyre doesn't leak air, use soapy water in a trigger bottle to spray around the rim, spoke holes and spoke threads.
There is fixes for a leaky rim, if you find one (or more) just ask on here.
Most two strokes respond well to having their silencer repacked.
The exhaust should have a mellowish bark when you blip the throttle and not sound harsh.
Undo the end cap, pull out the perforated pipe and gloopy mess and clean up the pipe.
You can get a preformed silencer insert for one of these or wrap the pipe in some silencer matting, but not too tightly.
Just wrap enough to hold the perforated pipe in place within the silencer, it should all stuff back in fairly easily, if not, it's too tight.
They do like the higher octane fuel and fully synthetic 2 stroke oil (at 1.5% mix)
Pointless adding more "just in case" as it leans the overall fuel/air mixture out and clogs the exhaust with unburnt oil.

If it's been stood a while, as already mentioned green furry crap seems to leech out of modern petrol and gum everything in the carb up.
The advice about draining the float chamber when left unused is wise, just shut the fuel tap off when you're finished with the bke and slacken the cap at the bottom (27) until it stops dribbling.
By the sounds of it, the inlet valve to the float chamber has jammed open, allowing fuel to poor in from the tank unrestricted.
As you've discovered, it pours out of the float chamber overflow and vent tube, but it'll almost certainly pour out of the float chamber and into the engine due to the fact the carb on these are tilted so far forward when mounted.
This will fill the crankcases with raw fuel, flooding it up.
First, as prompted already, clean the carb out well, if unsure about how it goes back together, there are plenty of Mikuni diagrams floating around the 'net like this
http://www.allensperformance.co.uk/exp-vm.html
Pay attention to the float chambers inlet valve and seat (34) and jets, particularly the narrow pilot jet (21)
Remove them and look through the small hole to verify they are clear, often you've no choice but to poke them with a fine strand of copper wire to clear them.
The green crap is stubbon, but don't be tempted to chisel at it.
The carb's float height is very important on these due to the mounting angle, as a high shut off height will allow the chamber to pour over allow the engine to flood up, too low and it pinks and bangs as it's starved of fuel.
Search the forum/internet, there's plenty of advice about setting them correctly.
Now that's sorted, remove the spark plug, lay the bike flat on the right hand side and repeatedly pump at the kick starter, I'll bet it fires black sooty petrol out of the plug hole.
Keep at it until this black petrol starts to clear up.
Refit the carb and plug and try starting again.