This starter motor may have been working okay, but when restoring a car it is good to disassemble and inspect everything, and fix anything that may appear to be less than perfect.

It appears the brushes are running rather thin, and the commutator could do with a clean up, but it is definitely rebuildable. Starter brushes need to carry about 200 amps, so they are solid copper (rather than carbon as you would find in a generator or alternator). The copper brushes are also soldered to the field wires, so it will require a substantially powerful soldering iron to do this job. The commutator can be turned down in a lathe, but if you can carefully chuck the shaft and spin it you may be able to clean up the commutator with a file as it rotates.

The pinion gear teeth don't look too bad, but it is a golden opportunity to refresh this part, especially if you have just installed a new ring gear on the flywheel. The first picture shows the new gear on left, and the used gear on right.

The commutator has been turned in a lathe for clean up. Outside diameter of the armature was polished with sandpaper while spinning. Original paint color for MGA was dark red like the engine, but all replacement units are black.

Refer to Workshop Manual section N.8 for information on servicing the starter motor. Also see
Lucas Fault Diagnosis Service Manual (2.4-MB pdf) in the Starters section, and
Lucas Starting Motors (3.9-MB pdf), Model M35G, and the
Service Parts List section N: Electrical Equipment for part numbers of the individual parts and assembly numbers.