If you are thinking of a ty 350 yamaha then carb jetting, Reed block with different stops, and a jug is all that is needed. I ciezed a pinky 250 when i was a kid doing something stupid the day before an event and swapped over all of the above from my 86 ty 350. The fly wheel is also different but i liked the rev of the 250 flywheel. The head fit from the 250 on the 350 head. If your talking about a 350 honda..... I dont know of one in the states.
--Biff