This is GOOD NEWS! If the old starter's the big heavy Delco-Remy, you can dismantle and reassemble easily. The Brushes are no challenge to put back in place because the motor end cap's a separate piece you can put back last. Drive, Solenoid, Brushes, Bushings are all available. BUT... The Field Windings and/or the Armature Windings, also the connections between Armature Windings and Comutator (copper segments Brushes run on) can fail. Field and Armature are major parts that affect the speed and power of the Starter. Check Brushes and Bushings. If Brushes are worn down toward the Screws that retain them, Starter's seen a LOT of use. If that's the case, the Rear Bushing could be worn out enough for Armature to rub inside Field. It's all magnetic in there, remember.
These D-R's usually can be salvaged, but WHY when High Torque is so inexpensive and performs better...?
Again, GLAD you have it running!