I will be watching your thread with interest. My 1961
Marvel coupler is complete, but it did not want to release the hitch ball on one occasion...so I am a bit leery of it.
I actually bought a new 2 5/16" coupler assembly to weld on in place of the Marvel, but my welder friend said to take it apart and clean it up...it should work fine after some love.
Supposing things don't clean up well enough, and I need to actually replace parts, I am curious as to what direction you gain here.
Seems to me a "universal" coupler repair kit should work. The parts are not unique to Marvel and have been the gold standard for decades. Fitment with a little modification, perhaps?