To package something like that inside of a G scale might be tough.

But let me suggest maybe using the OS30 FS. Its small, smooth, quiet.... and with a proper governor/throttle you can set what ever rpm you want. I have a OS26FS that is over 10 years old, and it still runs like a watch.

[link=http://www.osengines.com/engines/osmg0830.html]os 30 engine[/link]

You will want to run it below 10000 rpm, so you will need to govern it somehow.
Maybe use something small and lightweight like a [link=http://www.modelavionics.com/pro/]throttle jockey[/link]

If the genset does not have a lot of mass, perhaps add a steel marine-type flywheel on the engine... maybe incorporate a fan/flywheel combination for cooling ?? usually the marine flywheels were cut with a groove for a starting belt...which may come in handy as well.

Chances are with the type of electrical load you will likely utilize, you will never have to run that thing above 1/2 throttle. It will run all day on 6oz of fuel And running at that power setting, it will run fairly cool as long as you keep some air moving over it.

Saito has a small 30 size 4 stroke too. check that one out as well.

If you need higher RPM on the generator..... gear up from the engine rpm. There is pleanty of power available, and with a flywheel you will not have any torque problems under load. For your application, this is likely a whole bunch more practicle, and a lot quieter, than having a screaming engine in the cab.

Good luck with your project

(edits, added links)