I've heard Whizzers can be tricked out by punching a hole in the restricter, making them good for up to 65mph.
I had a Qinqi Happy 50 once, which I was "told" had 5hp, but it absolutely would not go up hills. I once looked into putting on a heavier flywheel, but the darned thing was welded on, maintenance proof.
What I'm looking to do, is build something super-cheap from scratch.
I'd like to start with a frame mounted motor, & maybe later on, add a front wheel mounted engine so I'd have a makeshift Rokon.