These used literal clones of the Yamaha TW200 engine as others mentioned, if you can find one for a reasonable price. I think Chinese Hondoid (nicknamed that because they are not really identical to any real Honda engines, but are similar) are good alternatives; very reliable!
Lifan or Zongshen will do the trick. Personally, I would go with a Lifan engine, but that is personal opinion, and not based on anything--in the end, either one will do the trick.