The only company that I have ever read about on these forums that has propane for fuel is Lehr.
The engines are 25cc and that alone is not much power for scooting an MB down the road.
Couple that with the fact that propane has less BTU's when burned than gasoline and the little engine is even more underpowered.
So far I have not read of a company that sells propane conversion kits either.
If one were to convert a Honda, Robin/Subaru, HuaSheng or Greyhound over to propane I would be interested in the results. Heck, I probably would follow suit and convert one myself.
The lack of a conversion kit is what stops me. I keep eyeballing the propane floor polishers at work for ideas. They have a horizontal Kawasaki single cylinder engine. What displacement I don't know.
Here's a site I found on the subject:
Small Engine Generator Conversion Kit - Propane Conversion Kits - Natural Gas Conversion Kits