servo drive circuit

Does anyone know how to make the circuit like the ones inside srevo motors? I am designing a sumo bot with radio control. I need to use contenuously rotating motors with lots of power to drive the robot. I do know how to modify my servos so they contenuously rotate, but I don't think they will have enough power to push around the other robots. I would like to use one of my 12v gearmotors instead of the ones contained in servos, but I don't want to destroy my servos to do it.

There are specific drives for motors that take servo inputs and turn it into high current speed control for big motors. People like Dimension Engineering
do what you need.