For a metal gear drive motor, easy to attach to wheels via servo horn, I modified a "no stop" servo motor.
Start by removing the four screws in the bottom of the case.
Next, cut the two wires from the circuit board that go to the motor.
Cut the three wires from the circuit board to the potentiometer. Remove the circuit board.
Take the two wires from the motor and solder extender leads.
Push the solder connection joints into the cavity of the servo motor housing.
Screw the bottom cover back into place.