Either solenoid or brushes. Test - just short both solenoid nut terminals with, let's say, pliers - if the starter motor starts to turn - solenoid - if no luck - brushes.
Condition of brushes doesn't really affect performance of the starter until contact cannot be maintained or contacts are brown enough from sparking. Alternative chance, when starter motor has been recently refurbished with aftermarket parts is that possibly brushes had excessive free play in their holders and one has become stuck. I've experienced that firsthand, solution was disassembly and brush holder adjustment, by making them tighter with pliers.
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