If you are talking about an ebay or made in China motorcycle or scooter, or one of those "pocket bikes" you are throwing away your money! They are absolute garbage on every level and don't last very long. After the sale service is terrible and the "warranty" is a joke.

I know, I've worked on a boatload of them.

I don't know how much we are talking here for the mini motorcycle, but I do know that these bicycle motors are under 200, plus a good used bicycle for 25-50 dollars and you are golden.

I have one for sale right now (engine on the avatar bike) for 125+ shipping (about 25-30 dollars).
Top speed 25 mph, 200 mpg, runs smooooth and is relatively quite.
Installs in under a half hour.

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If it ain't broke, and you mess with it long enough, it will be.