Heres some background to build your ideas from. A full scale IC engine on propane begins with liquid propane plumbed a convertor, At less than about 80PSI propane is a vapor, not a liquid. so a convertor regulates the pressure to a much lower amount so that the propane is now a vapor, it then goes to a carbarator of sorts, this device lowers the pressure of the propane, to a bit less than outside air pressure. This is so no fuel will flow unless there is a vacuum between the carb and the engine to draw the propane in.
You can't feed liquid propane to a gasoline carb, as it will boil off as soon as the pressure is less than 80psi. For a small engine that does not have a changing load, you could come up with a system that dribbles the liquid at the right amount to run an IC engine.
Beware of liquid propane, it will give you instant frostbite, also careful touching the devise that converts the liquid to gas as it also gets very cold.
