I did some RC car racing, back when I was 13. RC cars generally consist of:
1. A radio receiver / controller, tuned to the transmitter.
2. A motor or engine, depending on whether the car is gas powered (engine) or electric powered (motor).
3. A servo for steering.
4. A servo for throttle (Mainly for engines, or old styled mechanical electric motor speed control which is basicly a potentiometer)
5. An electronic speed control (Used for electric motors)
6. Battery pack, 9V somewhere around 1900mA or so. I think these are 6 "C" size NiCad cells in series.
The forward, backward motion is controlled by one motor. Inputs from the speed controller will reverse the voltage to the motor, causing the motor to spin in the opposite direction. To my knowledge, this is very difficult to accomplish with engines, unless you have an amazing transmission for the engine.