From what I can recall from days long gone by.....if you take a gas engine and put propane on it, it will run properly, but not as efficiently(power wise) as an engine built for propane. But when trying to make a propane engine burn gasoline there may be issues depending on how "built" for propane it was. If it was a factory built propane engine it may have higher compression ratio pistons than an actual gas engine. Gas engines also run "cooler" spark plugs if I recall correctly.
