When you pulled the starter apart, did it smell like burnt paint? If not, I'd be looking elsewhere for the problem. The windings on the armature are copper coated with a type of shellac to insulate the wire. When people overuse the starter by either holding down the starter button for over 20 seconds and not allowing the starter to cool off for a couple of minutes before doing it again, overheating will happen and the shellac will burn. The copper windings can short through themselves or short to the plates in the armature which is then a short to ground. Depending on how many windings are shorted to ground, it could be a partial short or a full dead short.
There are starter /alternator rebuilders that have better equipment for testing and I would be taking the armature to the rebuilder for testing before going any further. If the armature tests out fine, then the problem is elsewhere and I would be more focused on that first.
It's a 40 year old bike. Wiring is always a problem if not new. Copper will oxidize and copper oxide will cause resistance. For the wiring in this circuit, I would replace the positive feed wires from the battery to the relay, the relay to the starter and the ground wire from the battery to the frame and ground wire to the engine. ensure all contact points are super clean and use No-ox paste on all connections.
If you haven't disassembled the starter relay and inspected and cleaned, then you really must. The contacts must be clean and not pitted. Also the contact bar needs to be flat. When people over run the circuit, a lot of heat builds up in the relay contact bar and while the magnetic field of the relay is pulling in the contact and the bar is too hot, the magnet will bend the bar which reduces the contact area of the electrical contact. When that happens, not enough current will flow through the contact Also, pull the relay power feed contact "bolts" out and clean the heads up with a file and they need to be flat and 90 degrees to the threaded part. If you file them flat and they are not 90 degrees, again not enough contact surface with the flat bar contact to flow enough current.
The starter converts electrical energy to mechanical energy. Lack of sufficient electrical power in (and supplied to the right areas), no mechanical power out.
