The big difference is how the fuel and air are mixed. Older engines used carbarators, Now there are different carbs for liquid fuel which I'll use the British name for "Petrol" and real Gas (propane). However nothing in the rules say you can't have a dual carb setup one in front of the other
With today's modern engines the Petrol is not carbarated (Mixed with the air) but injected. Now, again, no reason you can't put a propane carb in the line ahead of the throttle plate, but I'm not sure how well it would work.
Next, there are dangers and concerns in converting, Diesel fuel can be dangerous, but you have to seriously annoy it before it gets all riles up and flairs in anger (Burns)
Gasoline (petrol) is much more easily annoyed
Propane is very easily annoyed and is stored under pressure, quite a bit of pressure, so you need a very strong tank to hold it, just incase some idiot decides to park his sports car under your fuel tank
Disel can be made to burn, if you work at it, Petrol very easily, but it does not often explode unless the fuel/air mix is real close, it may "implode" (The "Whomp" of a puddle of petrol ignighting is caused by air rushing in to the fire, not out)
But Propane sprays out in a cloud and can shatter windows for quite some distance when some idiot strikes a match so he can light his last ciggeratte (his very last, since it will be the one he's holding as he dies in agony)