The trick with the tiny little model aircraft engines is that you typically get a decent lift/drag efficiency multiplier with fixed wing vs. quadrotor. In my ex

The trick with the tiny little model aircraft engines is that you typically get a decent lift/drag efficiency multiplier with fixed wing vs. quadrotor. In my experience, there seems to be a somewhat scale invariant power requirement of ~200W/kg (applies to a 249g Mavic Mini and much larger 25kg units). For a fixed-wing, you can get a 5:1 or 10:1 L/D and get away with more like 20-40W/kg instead. You also probably don't want to direct-drive a multirotor off of the gas motor but rather convert it to electricity so that you maintain the rapid control authority on the motors. There are companies that make suitable powerplants, e.g. https://www.pegasusaero.ca/hybrid-advantage https://www.pegasusaero.ca/hybrid-advantage