thing about multi speeds, if your engine has a pretty broad range of torque, multi speeds will not gain much in acceleration bcs of the downtime req'd for shifting. ex. a v12 viper has like the broadest torque band of anything ever made lol and 1st + 2nd gear will get you to any desired speed up to over 120mph quickly. narrow high revving engines like say, a racing GP 125 motorcycle may have 9spd; only makes any torque between 12-16k rpm
so like a stock 66cc won't gain you so much acceleration but if you start adding a pipe, porting, etc then you can take advantage of perfect ratios. or if you plan on doing these things. 4 strokes not so much needed to have a shift kit + they can chew em up I hear,,