A comment on valves, heads and seats. Gasoline as a "wet" fuel provides some lubrication and cooling to valves. Propane does not. Cylinder heads need to be set up differently to live on propane. Clearances and seals for example. A propane engine needs more oil on the valve stem - meaning less effective stem oil seals - to live and be happy.

A long life propane/gasoline engine can be done, but lots of R&D or trial and error is involved.