This reminds me of something funky I did with an R/C helicopter quite some time ago. The engine is a 5.73 cc two stroke methanol-nitro engine. Platinum catalyst glow plug like most model engines.
To start the engine, you hold a 12v electric starter with friction fitting rubber against the plastic knob that is attached to the crankshaft. My friend and I had the crazy thought of trying to let the engine drive the starter as a generator once it was started and powering something with it.
Once we had the engine started, I held the starter on the engine while he hooked up a car headlamp on the starter leads. I clicked the switch and hit the throttle, well a little too much. I burned out the lamp. Something like this could be constructed into something the size of a 1 liter soda bottle.
Unfortunately, the methanol/nitro fuel is $16/gallon. The castor oil exhaust is incredibly nasty, methanol burning makes nasty formaldehyde smell and it is incredibly loud.
That small engine weighing much less than a pound can push 1.2 horsepower and about the right RPM for direct driving a small generator.