voltage variation when it's running should not be normal....go through the TM and adjust the voltage regulator. There are a couple of pots to adjust and the TM covers them pretty well...you may need to take it back out and hook it up to the simple test circuit though.
As to the DC side, are both batteries known to be good??? 25 volts running seems low to me...Mine are running at around 26.5 to 27 volts. Again, the TM has pretty good information about making a simple load test for the charging system, and checking the output from the generator coil, and voltage regulator. Just my
but if you find you'll need a voltage regulator, I would look on the internet and try to find a 15 to 20 amp 24 volt regulator somewhere. It'll probably be a whole lot cheaper than trying to but a NOS one, unless you can come across a good used one.
If you need more detailed information just ask, and I'll try to help out.
Edit: again, what do you have in mind for the old voltage regulator board?