Gabriel,

I would suggest that you first dissamble the engine and look inside what is the
condition especially of the conecting rod heads. What I mean is that those
engines have very(!) short life due to the extreme high rpm and thus heat
and there could be many reasons for you to not look for any model...
I had pretty bad experience with old 10cc OS max. I had to replace
connecting rod and crankshaft to get it working and the result was
pretty poor. But hopefully you are more lucky than me.

Cheers,
Nick