If you have it partially apart and you can measure resistance from the case ground to the windings, and they appear to be at the approximate value of the resistances mentioned in the previous post, windings are probably OK. If the motor base is together, with the planetary drive removed, you need to use the dish shaped nylon piece, which is the first item of the planetary drive outward from the phenolic plate with the brushes mount, placed over the shaft to properly support the armature shaft and center up the commutator between the two spring loaded brushes. Also have the end of the housing where the wires enter vertical oriented so gravity will keep the base of the armature in the lower bearing of the housing. Otherwise, the armature can't freely spin and can bind up. This below video link video someone posted does not have much useful info on the motors, but does demonstrate the binding up I describe if commutator is not kept properly centered between the spring loaded brushes. You mentioned a pad broken off inside, think it's part of the thermal circuit breaker ? That must have continuity for motor to work in either direction as it's in series with the ground connection, or motor case.
The motor(s) do require a stiff 12 volt source as either of the windings will draw a significant amount of current from your voltage source., about 4-5 amps. What are you using as a power source ? Battery charger ?