Jim,
You could always use the control board out of a cheap but powerful servo to use as a speed controller with smooth forward to reverse action.
You would have to add resisters to the servo board to give you "center" just like converting one for continuous rotation.
i am sure the control board in this servo could feed up to 2 amps to a motor and i doubt you would even draw 2 amps.
http://www.hobbyking.com/hobbyking/s...Fc07OgodjU8Alg