Phooey, I built my own from a 26” bicycle frame, using 20” wheels. I needed the extra room to alter the frame to allow room for a 3hp Briggs and a jackshaft reduction just ahead of the rear wheel. Took the pedal crank sprocket, and welded it to the small sprocket on the back axle. Welded the small sprocket to the right end of the jackshaft. Left side was pulley up to a centrifugal clutch on the Briggs. I never did put brakes on it.
I was 13 yrs old, but was advised by a mechanic that had made one himself. I sold it to a neighboring farm kid a year later.
Made a true mini-bike at 15, and a 3 wheeler at 17.
I sold the 3 wheeler (which I also hadn’t got around to installing brakes on) about a year after to the same neighbor that bought the converted bicycle. The kid’s dad said to me, “you don’t worry about stopping much, do you?”