Crazy but true...
During my pre-Navy college days I helped machine parts for a bores, stroked, blueprinted, knife-edged and balanced Briggs and Stratten 5-hp motor with EFI, 4 valves, and "smog pump supercharger." The contest rules stated you had to start with a B&S 5-horse, but any mod was game.
It was dynoed at 43 HP at 5000rpm.
The go-cart for the contest weighed 89#, with the motor.
Have you ever seen a go-cart lift the front wheels?