I've been playing around with pocket bike motor powered bicycle for a little while, check your local Craigslist for great bargains on broken pocket bikes, anywhere from free to $75.
I got 2 free ones, one with good motor, one with scored cylinder but complete.I also got a water cooled one with a bad crank(cost me $50 for a new crank) for work done on a second water cooled one. So there are lots out there to be had.
Also there's lots of speed equipment for these motors, some are Chinese clone of ADA and are very reasonable(MINI MOTO ADA TOP BIG BORE 4 PORT CYLINDER CIRCLE CRANK - eBay (item 230253318206 end time Dec-12-08 11:37:17 PST)
. These motors are actually 40cc(40mm x32mm) with big bore kit u get 48cc(44 x 32mm). I 'm running the 40cc one at the moment, it has lots of power ,I have one at 48cc with a ADA clone kit that has full circle crank 2 extra boost ports and claimed 14 :1 compression that I will try later on.These motor are nicknamed "Cag" motors.