OK, I just suspected that you may like to keep revs down. A lot of cruiser riders seem to get these bikes and can't abide going over 3k.
In town I cruise at 4-5k, a bit lower if it's a residential area at night.
I would recommend cleaning and retensioning all the bullet connectors in the alternator circuit. The ones under the engine side cover are often badly corroded from being at engine temperature, they all tend to overheat once corroded - too hot and the female metal loses its spring and won't maintain a good connection for long. The alternator circuit includes the harness grounds; fuse wiring; fuseholder; wiring to the ignition switch and back; the engine, rectifier, and switch plugs; plus the actual field and stator wires.
The fuses and fuseholder clips get corroded as well. After around 40 years, it's best to replace the fuseholder with a new minifuse type. There are ones available that plug in directly and use the original mounting bolts.
Once the harness is back to new condition the alternator should be back to full output. You should measure within 1V of the battery terminal voltage from regulator white to ground with key on and engine stopped. If you still don't get charging - do you have any extra electrical loads? High wattage headlight? Driving lights? If not, suspect the regulator and rectifier. Bridging the rectifier (connect black and white wires) puts the alternator at 100% possible output. The system voltage should get above 15V at 4000rpm rapidly after kick starting on a fully charged battery. If that doesn't happen then the rectifier should be carefully tested, possibly replaced with a known good substitute.
Electronic field control regulators are fine, but a working electromechanical one is equally effective. Avoid shunt regulators: the alternator was not designed for the extra heat produced when using these.
There's no magic available to make more power with an electronic regulator
The physical coils are robust and I've yet to find a failed one except from crash damage. The rotor is a dumb lump of metals - nothing can go wrong with it unless it gets smashed or someone has removed too much metal in search of a lighter flywheel. I ran into one where an owner, having read about of angular momentum, lightened the rotor by turning down the outside diameter - removing metal there maximizes the reduction in flywheel effect. That killed the alternator: the rotor works by warping the field coil's magnetic field and then presenting it quite close to the stator's pole pieces. The gap must be as small as possible.
