I'd tear it down and inspect/test the Stator, field, commutator rings, and rectifier Bridge. If there in good condition, a simple clean-up of the commutator rings (with the proper glass bead sand paper) and a new set of brushes/voltage regulator should do it. If the vehicle has mileage, I'd also recommend new bearings.
The brush/regulator and bearings should be stocked at most auto parts houses. The rest might be special order or dealer only.