What kind of motors are these? Series DC, Permanent magnet DC, AC single phase, or AC three phase?
DC motors and AC motors have different things to look for. If these are DC motors with brushes it it usually a good idea to replace the brushes and true the commutator. Megger is good on these as well as on the AC motors. Also, on both types check the bearings for wear and play.
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If it is broken, fix it. If it isn't broken, I'll soon fix that.