The starter/generators on these classic old machines don't put out alot of current. I don't have a manual nearby now but I'll swag 10 to 15 amps is all you can squeeze out of them. Good news is, this is plenty to run the tractor and run a couple lights in the dark too.
The charging system needs very little maintenance. I'd only recommend to adjust the regulator ONLY if it isn't charging.
Take a multimeter (volt ohm meter) and read the voltage of the battery with everything off. You should see anywhere from 12.0 to 12.8 volts.
Start the tractor and take another reading, you should get a higher reading at 1/2 to WOT rpm.
If you read 12.1 before start you should read 12.9 or higher at 1/2 to WOT.
Readings of 13v - 13.5 volts is OK too. It means the battery is really low and the system is really putting out the current to charge it back up.
If the voltage remains the same or lower after starting it up, you'll need to adjust the regulator. Keep in mind that they won't charge at low idle and don't be alarmed if your chargeing lamp comes on at idle. Just bump the rpm up and the lamp should go out.
Make sure the battery is fully charged and if it's over 4 yrs old you'll probably do better just to replace it. These old machines like fully charged batteries
As an FYI:
The starter/generator and regulator on my 1966 856 are still the original OEM. I had the starter/generator rebuilt w/new brushes, bearings and the commutator turned for around $20, 10 or so yrs ago but it's still the OEM.
The ones on my 1966 876 look to be original but in it's case I'm not 100% sure as I've only owned it around 5 rs now. In any case that'll show you that they work good and last along time.
If you do want to purchase a spare regulator make sure it has 4 terminals. Theres one hidden on the bottom that you can't see. Odd's are you probably don't need one tho.
Good luck and let us know.