Usually when an alternator has a weak output, I remove the brush holder (usually attached to the voltage regulator) and find the brushes worn down to approx. 3mm length. One is usually worn more than the other.
I go to my local parts store and find replacement brushes for any alternator (Mitsubishi, Denso, etc.) separately for about $10-$30 (£4-£12). If they are slightly too fat, they can be filed down to suit. Then it is a case of soldering onto the trimmed end of the old braid.
I also give the rotor a spin and if the bearings are dry-sounding, an air impact gun (rattle gun) gets the nut off. Some careful prising with levers extracts the bearing - if really stuck it can be cut off the shaft with a grindr. Bearing shop provides new bearings, standard size, about £10-£15 each.
If I'm feeling generous I'll paint the housing too. I've dealt with 156 and 164 alternators (Bosch and Marelli) in this way, plus about a dozen alternators for FIATs, and only ever had one faulty diode once, which was a pain but I managed to replace it for $3. It's surprising what you can do if no replacement alternators are available
Similar story for starter motors.