Consider where you want to "zip". The 50cc bikes will hit a top speed of about 35mph (flat road). You might be able to squeeze another 5 mph by de-restricting the exhaust. The 150's will go 50-60. Anthing above that and you're pushing the limit. The Buddy's will go about 5 mph higher than most similar displacement bikes and if you can find a 2-stroke engine you can pick up a little more.
Spend an hour at Scootdawg.com for good info on the China bikes.
