A company in Japan makes the 125cc DOHC kit for 50cc pit bikes, which are similar Honda based engines, I was lucky
that it was a virtual bolt on kit, it , consisted of crank/rod assembly, cylinder and piston and DOHC top end,
its high end stuff and rather expensive and recommended for racing only, although I've done a lot of touring miles two up
and has never been one issue with it, it revs like a 2 stroke compared to my other one which is bored to 140cc big valves, cam etc. I was considering converting the engine from 4 speed to 5 or 6 speed and using the engine to be the base for a early 60's Honda racing bike project, kinda retro, I've been gathering bits for this project , just to many projects and not enough hours in the day.