Please God not the starter clutch..

After spending hours fixing leaks and setting up refurbished carbs earlier this year, The 550XJ ran fine until last night. In the heat, it refused to start and the starter sounded like it was struggling. I also got some of the dreaded "box of marbles" sound as the starter clutch tried to engage, but the starter motor itself seemed like it could not spin properly. Bump started it and took it home. Now that its off the bike and opened up, this is what I see...

Seems like a few others have had this issue, based on my perusing the forum. The small diameter gasket mated to the engine housing was OK, but that's a lot of oil! Luckily I do have a spare starter.

I fear now that I may have overfilled the bike during its last oil change. Back when I rebuilt it at 54,000 km, I cleaned the oil filler window best I could, but we've doubled the mileage since and the window is back to opaque making filling from an 18L jug subject to guesswork. My next steps are to refurbish and test my spare starter, drain the oil, and do a Seafoam "rinse" with new oil in the hope that the starter clutch is still OK.

BTW, here is what the armature looked like once cleaned.
I doubt this one is refurbishable. That's what 41 years and 108,000 km of street riding can do I guess.

I read the starter motor rebuild thread, so I'll proceed to tear down the spare starter and refurbish that one (18,000 km?) Looking for suggestions on best way to test it before installing and best way to clean out/off the starter clutch, beyond a seafoam rinse. Oil is Rotella T4 15W40, btw.