Variable speed is added by varying the voltage to the motor however, at 6 volts operation the power is reduced. The unit that I have has a gearbox at the front end...that would be a better solution, maintain the same power, reducing the speed by gear selection, like some of the portable power drills use.
Yup, good approach.
The thing is, I don't think I have anything I can scrounge from laying around.
I may end up needing to buy one but hand held and with variable speed would be nice.