RE: Using Radio Control to control other things??
Ideally you should start with a self-propelled powered lawnmower. That way all you have to do is actuate the throttle and steering using servos. That's an oversimplification obviously but that's the direction I'd take. Basically the radio control system simply allows for you to control the servos - what you do with those servos is up to you. You can use one or more servos to actuate any linkages or controls the engine and carb may have, and use one or 2 powerful servos to actuate the front wheels.