If you are talking about a bench grinder, I am confused, as 90% of the tool is the motor, and wondering if your friend gave you a pile of shields and abrasive wheels.
If you are talking about a handheld grinder, I am confused, because the motors are integrated into the case/handle, and not something you replace off the shelf. Wondering if your friend gave you some garbage.
If you are just talking about brushes on either of these induction motors, then that is less confusing.