Sounds like a neat idea, how about this:
Use some high RPM high load DC motor. Like a RC car motor. If you can find a DC motor spec'd for the gas motor's RPM, great. If not the gearing for that high of RPM will need to be pretty good, although maybe you can use a gearbox out of a gas RC car? Use a flexible coupler for the DC motor shaft connection.
Hook the leads of the DC motor to a gel cell battery through a switch.
Use a SMPS PWM IC to output a PWM signal to a RC servo motor. Connect this servo to the throttle control. Set the throttle to only go down to idle, and you'll also need to limit the max RPM somehow I'd guess.
When you need to start it flip the switch. This will use the gel cell to start the motor, and when it gets going, it will self regulate with the PWM IC.
The DC motor efficiencies should be pretty good I'd think, although I'd shoot for 50%. You'll probably need to heat sink the motor, maybe put a fan on it. A fan would be great for the gas motor, as it is used to moving air across it's fins. Also you'll need to find the right DC motor winding for your application.
You could even come up with a glow plug circuit, and auto start/stop.
Just some crazy ideas, who knows if any of them will work..