Check you haven't got an air leak between the carb' and engine. Start the motor and spray something like WD40 round the area to see if there's a change in the revs.....
Details on here Colin.... http://www.cubsite.com/Spares.htm
I had the pleasure of trying a trials Cub at the weekend with a diaphragm clutch, what a revelation.
I've got that model in my collection, my mate Andy borrowed it last July to do a charity trial at Mallory Park and fell in love with it. They're a lovely bike, you'll enjoy it. And they've got a seat....