As h.240 said, it is the solenoid. If the starter spins but doesnt engage, the solenoid is most likely the issue, not the brushes.
If it's worn out to the point that the solenoid isnt passing current, might as well look into a new one to be safe.
If you have a multi meter, you can check this to see if the solenoid is still functioning. I mean... hey, I picked up a starter on nico for 25 shipped with less than 5k miles... sweet deal or what?