I have no idea how the pin got lost, but I noticed that the mower deck was no longer moving up and down with the 3-point hitch. l started to look around to see what on the back could control what on the mower deck and eventually I found something that didn’t seem to be connected to anything with a hole that seems like it would match up with something near the hitch.
I was able to pore through the parts diagram online (https://www.messicks.com/ku/90312?sectionId=229624&diagramId=458209) and found that I needed part 70 (the pin), a pair of washers (80) and the cotter pin (90). I could have used any old cotter pin, but I figured since I was paying more for shipping than the parts I would just as well get the right part.
And now it works! :-D
Ok, so I looked online and the ring is just chump change… but whatever. At least I have official, complete with hologram, Kubota parts on the tractor.