When/If a motor had been overheated, the impact varies....
Some Magnets are much more tolerant than others (Reedy MVP's always seem to get HOT, but once they cool down, the magnets are as good as ever)...
The most common "damage" is that the brushes get hot and turn a purplish color -- when that happens, they are ruined, as the lubricant in the brush has been cooked out and the copper and silver components have been "annealed" (the metal actually changes it's characteristics)... Over heated brushes MUST be replaced.
Similarly, the comm will turn colors , ranging from copper to brass to red to purple. As with the brushes, this can harden the copper. The heat cycles can also make the comm out of round, as the copper surface and the phenolic center core will expand/contract differently from the heat. Carefully skimming of the comm on a comm lathe can "mostly" revive an overheated comm (the copper may be permanently hardened/changed by the heat). Skimming the comm will be "good enough" for most casual use -- hard core racers would find the results unsatisfactory, however...
"If you cannot win, make the one ahead of you break the record."
Biff Racing Team #420 (Ah... The "good old days"...)
The local indoor Offroad Track: The RC Race Barn
TLR22-3.0, TLR22T-2.0, TLR22SCT-1.0