If you go to 600cc you will loose rpm at the top,but you will have everything at the bottom with 600cc 14/42 about 100 mph,stock and 14/42 110-115 mph.We have done with stock and port.cyl. bigbore carb. to 41mm,v-forcereeds, 14/42 topspeed 120 mp, 15/42 130mph.
There will always be problem with 600 cc becuse you have to modified the crankhouse and to big pressure on crankshaft and bearing.