The YS is total a non starter because of the silicone parts in the fuel system. The Saito is possible but you will be trading HP for MPG.
I have done an ENYA 120 to gas fun project to make it right, but the carb, ignition and oiling sysem cost as much as the engine.
Have you looked at the Stihl 26cc 4C engine its supercharged which helps a bit with the power. You are only looking at 1.5hp.