I bought three of these 14.4v drill from a local Harbor Freight store when in New Orleans in the summer. I'll keep one for now as a useful drill/driver and convert the other two into gear motors for use in the Kits

The drill is a model 40209.  550 RPM with a 36:1 gearbox. At 14.4v this give reasonable speed with 4" wheels but if you are using 3" wheels you'll need to go to 16.8v - 18v to get good speed and power.

The chuck is secured with a left hand drive screw in the centre. If you are lucky this comes out by turning it clockwise. On both my drills I had to destroy the chuck with my grinder and dremmel a slot in it to get enough torque on it. An impact driver may help here....keep the screw!

The remains of the chuck were then removed by clamping a vise grip on it and tapping it anticlockwise to loosen it. Continue anticlockwise to remove.

With the chuck off dismantle the case by removing the numerous small screws.

The case can them be pulled apartand the Motor/gearbox removed. Cut the leads off near the switch.

The clutch is removed by unscrewing the two screws shown. It comes off and lots of springs, ballbearings  revealed. All the loose parts can be discarded.

Three of the popular sizes of Harbor Freight  motors can be seen here. Note the 12v gear motor in the middle has a slightly longer shaft than the 9.6v or 14.4v versions.  This may be due to the type of chuck (cordless or key) on the originals. Otherwise they are essentially physically the same.

The clutch needs to be locked for the gearbox to work. This is done by screwing four 10-24 set screws equally spaced into the holes the ball bearings came out of. Do not over tighten them. I use mcmaster part 92505A235

There are some plastic features on the gearbox casings which I remove with a hacksaw and file so they fit in my Kits.

I then fit a small capacitor and some 12GA "Wet Noodle" wires with 30A Powerpole connectors. The +ve connector is marked on the motor if you look carefully.