A servo consists of a potentiometer, motor, geartrain, and electronics. The potentiometer is attached directly to the output shaft and changes resistance as the output moves, allowing the electronics to determine position and power the motor. You have the skills to build your own geartrain, use your own motor, and add your own potentiometer (you can use the stock one or any other potentiometer with the same value). The problem is that a servo's pcb (printed circuit board) is built to control a small motor and it cannot handle the amps a larger motor pulls. The fix is to remove the stock motor, replace it with a suitable h-bridge, and add your own motor. Adding an h-bridge will also allow you to use power from the main battery pack. [link=http://www.webcom.com/sknkwrks/mowers.htm]Here[/link] is a writeup on the process. If you tell us what size motor you plan on using we can give you an amp value to aim for.