I used to be real big into minibikes when I was a kid.
I was always strapped for cash and I made a franken briggs with a cracked block, unknown rod and piston and a screwed up crankshaft. The piston sat at least .100" down into the cylinder, which I never honed. VERY low compression. It ran, it wasn't fast, but I ran it for at least a couple months.
Replaced that engine with a 5hp Briggs I got for free off an old front tine tiller. Never changed the oil, just put it right on my bike. It was a good upgrade, but the engine had a LOT of vibration. I was driving down an old county road and the bottom of the rod shot through the block when I had it pegged down hill with no governer. It's amazing the abuse they'll take.
Also had a Honda GC160 OHC bust a valve, hit the piston, and punched a hole in the head. Left a long smoke trail from burning oil, but was a powerful and smooth engine. It would have cost more to fix than to buy another.
I started on a OHV Briggs Intek build with Animal internals and a steel cam, but never finished it. Those OHV briggs are bad ass little engines, power stock wise, they really put the old flat heads to shame. But they do have some insane aftermarket parts for flat heads.
Pete