Summer of 67, my cousin and I (ages 14 and 13) built a mini bike from a 26” bicycle frame. We rebuilt the middle part of the frame, and used 20” wheels to give more room for the 3 1/2 hp Briggs. A jackshaft was mounted just ahead of the rear wheel with the original sprockets trading places. On the left side there was a belt drive centrifugal clutch, and a slightly larger pulley on the jackshaft. No brakes, but we didn’t care. We had a lot of fun with it that summer, and then sold it to a neighbor farmer’s son. They kept it going for another year or so.
That sounds like a sweet set-up. If I had something like that at that age, I would have spent my summers wandering all over creation!