Here is an update, I am still looking for a rebuild kit and/or a replacement starter Kubota part number 17341-63010.
The continuing saga of the starter. It now works, spins the motor, everytime.
I took it back off, took it to the bench. Tried connecting a battery directly to the terminal going to the field/brushes. No movement. Took the brushes out, within tolerances. Took armature out, lightly sanded the plates. Sanded the commutator again. Put it back together again. Tried the battery again. This is a battery I just moved indoors from the trailer. No go. I was ready to scrap the starter. I tested the volts on that battery, 10.xx volts, not enough to drive the motor. I grabbed another battery I just moved indoors, from another tractor, this one has 13 volts. This one spun the starter motor.
Took the cover off the magnetic switch again. Cleaned the contacts again. Tested the ohms between different points, discovered that the plunger contact was not touching the input terminal contact, unless plunger contact was pushed very very hard down.
I removed that contact, put it in a vise and gently bent it up. Reinstalled the contact. Tested the ohms again. Seemed good to go. Put it all back together, installed in tractor, tried it.
I turned the key and it hit first time. Turn it off, tried again, it hit first time. Tried again same result.
I think I got it.
For the time being i have a working starter.
I definitely must find a rebuild kit or a replacement starter.
Boyde