The things to watch for are clearances between tyre and mudguard, the front especially. Also the speedo will read lower than you are actually going (Imagine the embarrassment, a speeding ticket on a moped!) plus gearing will need dropping, say one tooth smaller front sprocket. Or you could go really crazy, join the C90 club forum, there are guys on there that have modified SS50 motors to 108cc without any visible changes to the external appearance of the engine!