Iain Mac Donald wrote:

The copper (?) coating on one segment is gone but looks more like
cosmetic damage than anything major. Do you think this is significant?

The segments should be solid copper - you could try polishing
the commutator with a dry cloth and some meths, and ensuring
the grooves between the segments are clear - don't scratch the
commutator surface, or undercut the segments.

What does the discoloured segment look like?