If you have access to all the tools to handle re-fabricating it into something like you want, the $500 50cc scooter is a pretty good parts deal: a 50cc engine w/3-speed auto gearbox, PLUS a dynamo and full lights, and a bunch of other parts also. Plus, if you can find one with wire-spoked wheels, then you've got solid hubs and brakes also that can be re-spoked onto other (larger) rims. ...Or just buy some (Chinese) dirt bike wheels,,,, the dirt bike/pit bike wheels are still wire-spoked usually, and most I've seen pics of had 36 spokes. Ya just need a different sprocket....
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The China girl bicycle motors will never be built that well, as there's not enough consumer demand anymore. The 2-stroke engines are already banned in China. The quality might hold steady for a while, but it is definitely not going up from where it is now.