I forgot I actually installed a couple of the Cadet 3s at our last place. They were pretty low maintenance and didn't really give me many problems. So maybe I'll suggest those.
edited to add--- I'm just providing info based on my experience. You've probably done more research into proprietary design of valves. You've got me second guessing whether a new valve will be available 10-20 years from now. Care to share what you've found out so far?
I haven't really researched the canister flush valves that much - maybe I'm just not familiar enough with them:
https://www.youtube.com/watch?v=cWosIAP7xHkSeems pretty easy - except, I didn't want to break the one in my parents' tank so didn't fully remove it. I actually started trying to remove it but then I heard it "snap" and wasn't sure if that is a normal snap or a "I broke it" snap so I turned it back into place and left it lol.
Anyway, after looking into it more, I think the canister valve itself may not be the problem at all and it may have to do more with the toilet just being old and mineral deposits building up inside of it, causing it not to sufficiently flush. The solution one guy came up with on the thread I was reading was to basically use makeshift tools (bent screwdrivers, etc) to poke around inside the flush hole between the tank and the bowl, and try to chip away mineral deposits that were clogging up the main jet which was causing the low flow of water into the bowl. And I wasn't about to start going down that path lol. In the meantime, I just told them to hold the lever down until everything is flushed.