I got two Pittman geared motor at home, they're from my old Nexstar mount.
My impression is that they're of 1/4" or ~6mm shaft, i.e. very suitable for my project, therefore, I tried to check its speed. On applying 2.4V to its terminals, it won't move, but I could feel there's some response inside, but there's certain no visual movement. On applying 6V to its terminals, it moves pretty quickly, probably too quick for autoguiding kind of movement, however.
Since it won't move at 2.4V, I guess I will need PWM to control its speed, and that will complicate the circuitry a lot, and so I guess that I will continue to look for alternative motors.
On more careful testing 2.4V is enough to turn the motor, and the speed is not too fast, however, I checked that the shaft is too small... maybe like 5mm or similar.
I've also tried to hand fit my existing tiny motors to the focuser knob of my C5 OTA, and I guess it's not too hard to put them together, maybe I can use one for them to motorize the focuser of my C5.