Thanks for the replies folks, they got me what I needed. Here is an update on my project.
I was able to source a rear bushing locally using the sizes you provided in your reply, thanks for that. That was the only one that was actually worn out (and it was worn out badly). The local part number (in case anyone needs one in a rush) was SS2024-12 Oilite 5/8 x 3/4 x 3/4.
My starter tested out fine electrically as per the tests in the manual and the brushes were long enough, so I replaced the worn bushing, dressed up the commutator, cleaned everything and then reassembled the starter. Once back in the car that starter worked better than it ever has in the 35 years I have been driving this car. My guess is that the worn bushing allowed the rotor inside the motor to rub against the cores of the field coils and the mechanical friction slowed the motor a touch until it eventually became noticeable.
Anyhow, the car starts instantly now, even when the engine is very hot.
Thanks for the help on this.
Adrio