Unless you love spending money on stuff you don't need, test the coil first. The yellow wires should have the same resistance between all pairs and no connection (infinite ohms) to the case. The actual resistance spec is in the shop manual - remember most meters are inaccurate at low Ohms, a reading close to spec and equal across the 3 measurements is what you want. The field coil resistance should also be checked - also ensure the rotor is correct, it was not unknown for POs with a lathe to "lighten" the rotor/flywheel by removing metal: too much removed and the power output drops off precipitously.
The #1 cause for bad charging is simple overloading with high power headlights, driving lights, heated grips, ignition systems, etc... #2 is corrosion on the harness connectors, fuse contacts, and switch contacts. A thorough check, clean, and tighten of all these is required every 5-10 years or so - and that was probably never done. Bullets where the clear plastic sleeve has turned black from heat may be permanently damaged because the metal has lost its temper from overheating. Any with charred sleeve, insulation, or conductor must be replaced or bypassed (with soldering or (less excellent) a compression splice).
