There is one other problem -- most states won't let you register it as a motorcycle unless it has been built on a motorcycle frame, and has other components tha

There is one other problem -- most states won't let you register it as a motorcycle unless it has been built on a motorcycle frame, and has other components that meet the appropriate safety standards (and probably for good reason -- see other comments regarding brakes). Of course, that makes the whole bike heavier, which requires a bigger motor/battery, etc.