I would try and get hold of a GEBE mount kit & sheave secondhand and then get a tuned engine from davesmotors.com. GEBE kits come in either 52mm or 78mm clutch sizes.
you might have to look for a while, but that would be the best of the best.
Other options include a 2 stroke CIF kit such as the dax 2-strokes and such and then get a shifter kit to go with it, so you would have a manual 5/7 speed motorcycle.
It depends if you want bombproof reliability - because if you do you will need to pay a premium.
hope that helped