RE: propane weedeater engines in planes?
Usually propane comes in a high pressure container which could be an issue for a model airplane aplication.
All engines require lubrication, not knowing my self about those engines I would think that those engines should be heavier because of an included lubrication system.
Anyway this is a hobby and we all experiment with many things that are handy, just take care of those containers under pressure.
Good luck.