Since LP gas (and possibly CNG) has a very high anti-knock rating (around 110 octane) engines that run that kind of fuel need around 12:1 compression to get the full benefit from using it. The exhaust valve seats also need to be made of a hard material like Stellite to avoid erosion of the seat due to the "dry" fuel. Once an engine is modified that way, running gasoline, even temporarily, isn't an option. Some farm tractor engines were purpose-built to run on LP, but I haven't seen a new one set up that way in over 30 years. Diesel is a much better option, in my decidedly biased opinion. "Your mileage may vary"!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!