Remember that, though it appears that the collar holds up the rudder, the tiller head is the real safeguard against losing the whole thing into the sea, because it's the only part that's drilled through. The collar just provides a bearing surface. On my boat the fiberglass plate underneath the collar was chipped and loose enough to be taking on serious rainwater, so I had to jack up the rudder and rebed the plate with 5200. Just recently I gave this over, scraped out all the 5200 and epoxied the plate to the cockpit seat. It's all faired in like a proper molded-in bolster-- because there's no reason it has to be removable. I also re-'glassed and faired in the bit of rudder tube sticking up through it so the collar rides on an even surface.
I am crossing-drilling the collar to take a long 1/8" or 3/16" clevis pin through the whole rudder shaft.
You might browse McMaster-Carr (mcmaster.com) and see if they have any product that will do for a replacement collar. Acceptable materials might include phenolic plastic, very hard (80-durometer and up) rubber, sufficiently-stuff nylon and some fiber-reinforced composites. Don't count on finding an exact match; but so long as you can tool the I.D. to match the stainless-steel shaft, and retain a snug fit, the rest is only cosmetic.
