I would be more worried about weight redistribution and wing loading. I believe you can add another servo to the radio for the throttle. That's not much of an issue. You would have to use the same battery pack because the ESC is built into the rx, plus the extra weight of the engine, mount and fuel. more speed will be more stress on the wings. If you have to beef up the spar, that's more weight. Nitroplanes has a cute little .10 size cub that would probably be a better choice for nitro power.
http://www.nitroplanes.com/newj3picub10.html