To determine if the brushes are in trouble, pull off the field wire from the alternator and run an improvised positive wire from battery positive to the field wire terminal on the alternator. Do not connect it just yet.
Now start the motor, let it settle and you will see no output voltage at the battery. Momentarily touch the makeshift wire to the alternator field terminal. Be careful. There should be an audible whine as the alternator comes to full load. The belt will object too, and often it will slip and smoke, so make the touch momentary.
If the alternator does not respond to that, then I would suspect brushes or rotor. If it does respond, then suspect the regulator. You cannot fix them. You will need a new one. Check the terminals and the earth on it.
Stop the motor, and check for reasonable continuity between the brush terminals. The last one I looked at was open circuit, but if correct, expect about 8 ohms there. Open circuit was dead rotor for me. A broken brush, or broken brush terminal will be obvious.
This subject is topical for me just now. I have a Russian motorcycle, and the lights are flickering again tonight. That's after finding a shot rotor on the old alternator and putting a new alternator on there. It looks like the regulator has gone too. Either that, or I have a rogue earth. An analogue multimeter will betray it, in the moring, I hope.