Concerning that mini bike,..you would be seriously amazed at what a 3.5 HP lawnmower motor will do if you over-ride the governor on the carburetor and replace the muffler with a straight pipe.
Some motor head guys I knew in high school bought some kid's go carts with 3.5HP Briggs and Stratton motors and did that to the motors. There was a church across the street with an asphalt parking lot and they would bring them there to goof around with them. The carts would go fast enough to make you pay attention. Taking a sharp turn while wide open would drift the rear end of the cart on asphalt.
It doesn't take long to blow up a lawnmower motor turned loose like that, however. They would blow oil through the crank seals, also,...so you had to keep an eye on the oil level.
