I run propane engines at work for industrial use, we start with standard GM engines and make them dual fuel or propane only. We only run them to 3000rpm and they actually produce more power than gasoline at that speed on some of the engines. The issues with higher speeds are higher exhaust temperatures since you can't just add more fuel like you can on gasoiline to reduce the exhaust temperature, and getting a system that will have the correct air fuel mixture for all speeds. I doubt the EGT would be an issue on these small engines.