Just have it run in addition to the engine. You still reduce fuel use, don't need to change any other parts to work without the engine. You just basically nee

Just have it run in addition to the engine. You still reduce fuel use, don't need to change any other parts to work without the engine. You just basically need to know when the accelerator and break (and perhaps clutch?) Were pressed, and perhaps change accelerator response to make it feel more natural.