It is best to spend a few $hundred on a modern C melody mouthpiece. I have a Morgan and it really opens the sound up. The old C mouthpieces are VERY stuffy. For a vintage C mouthpiece, a Bass Clarinet reed is the closest. You may need a #3 or stronger with the original mouthpiece.
I have kept one original C mel mpc.
I use it for xmas carols, and other quiet music in the parlor, with the piano.
As long as I play softly and do not try to push it, the results are very good.
I would agree completely that the Morgan is a splendid mpc for the horn.
However, if one has such an old original mpc with a decent facing,
it would be worth a try to see if it can be used in this way.
I have heard the C mel called a "parlor horn" and my old original mpc works well for that limited purpose.
I think the tip opening is very narrow --- around .060, perhaps.
The cheap plastic chinese C mel mpc I got was not bad, either.
In addition, the Bb metalite works pretty well when volume is wanted.
There are options besides the Morgan which can be used if money is short,
depending on need.
But the Morgan is really good.
These work well for me: