Welcome to the forum!
Skull is pointing the right direction and sounds like you are on the right track 4.1 is pretty low.
First, my guess is the gauge bulbs are burned out, that's real common, 'specially if "some guy" ran with a bad or no battery, voltage spikes etc. LED replacements are available the later bikes use bayonet style gauge bulbs. Does the tail light come on? Blown bulbs back there also pretty common. Good ole 1157's Don't rule out ground issues, the battery ground strap can be rotted. Some PO's can really trash a wiring harness cluelessly trying to fix electrical issues. And hey these things ARE 40 years old, rough storage, can create connection issues with various connectors,.soak with silicone or wd spray, THEN open, clean reassemble, all your electrical plugs.
Get a voltmeter on it, tell us what you see off, on, running? If it's still stock ignition, it generally won't run long unless there is some charging going on. The TCI ignition module likes a healthy 12 volts. Sounds like you have seen some of the charging threads. Check your brushes, they do wear and get too short to make good contact with the slip rings. Check all the fuses and the fuse holders, they tend to rot from age.
Good chance you are dealing with the issue that killed the PO's enthusiasm for the bike. But it's seldom hard or very expensive to FIX the charging system if you do correct troubleshooting. Lot of guys get all it's too complicated and go spend $600 on new parts but that's seldom needed. I've done 30 plus charging systems never needed to install an aftermarket, though I AM doing a vape charge/ignition on a custom I'm
slowly building. If a new rotor is needed,
@Jim here does rewinds, probably the best deal going.
The late model solid state regulators do die from age also. So some time with a VOM tracking down the issue is in your future.
Always helps if we see some pics of the bike helps get an idea of what issues you may be having, plus we just love pics!
Fuse box area, inside the LH cover. Well that's the short list LOL, keep us in the loop? If you go at this right, it'll be worth it....