Don't bother.

You'll need to build an electric remote starter for it first off for it to be allowed. Secondly, you'll need to add a flywheel and clutch, otherwise your weapon will have a lot of trouble getting going owing to the torque curve of an internal combustion engine starting off very low, while the starting torque of an electric motor is as high as it'll ever be throughout the rpm range. This also slows down your spinups as you will spend a fair bit of time in the low rev range when you aren't making a lot of torque. Nitro engines will need a lot of reduction too as they rev to silly rpm.

If you want a super-spinner, go brushless/lipo... it's more powerful per weight anyway.