If I have correctly understood all the facts, I frankly don't see the problem. Just order a couple (one for a spare) new 5/8 -18 (for a 7/8" or larger barrel) long tang breech plugs from Track of the Wolf (TOTW) and cut and file one of them to the shape you need. Those tangs are about 7/16" wide which should be more than enough for you. The tang at the breech of the barrel should be filed the same width as the flats which should be no more than 3/8" for a 7/8" barrel. The tang you get will certainly be more than wide enough to fit a 7/8" barrel. From your picture, It looks like you should have enough width as the tang tapers towards the back. If for some reason the tang isn't wide enough, just forge it out wider where you need it, but that doesn't look necessary to me. As an example of what you can do, I recently forged a 9" long over the comb tang out of one of those long tang breech plugs from TOTW. If I can get that out of one, you can get what you need out of one. It is 12L14 steel and real easy to work.
As to fitting the breech plug, you do know that you are going to have to shorten the breech of the barrel as well as the face of the breech plug? Any time you re-breech, you are going to loose up to one thread; in this case 1/16". When I re-breeched the barrel for the plug and tang mentioned above, it cost me 1/16" to index the plug and tang at the same flat as the old one.
Also, once you get within a quarter turn of the final position of the plug/tang, you have to take things real slow, breeching a barrel by hand with files is very, very tedious. You want a tight fit all the way around the plug face and it should be flat and polished bright. I finish up with a #2 Swiss barrette file.