An air cooled KTM 50SX engine. There are plenty of clones available from China. they are compact and make almost 9hp. You would have to fabricate a mount system, possibly modify a pipe to fit the bike, and figure out the ignition wiring (not hard, but still a thing). Then it's also a matter of matching gearing to the bike and the engine. They use a wet centrifugal clutch and drum bell, and you can modify the clutch wia different spring rates to change the engagement RPM and keep it in the power curve. Link to one: https://www.aliexpress.us/item/3256...=scene:pcDetailTopMoreOtherSeller|query_from:
This is also far from the cheapest option. It will cost at least $400-$500 to get an engine and carb. Then you still need everything else to go with it.
I saw somewhere that I could do a over 49cc bike if it has pedals is this correct?
also I live in a semi small town so maybe it would be easier to show cops that it is not illegal.