Track of the Wolf is one of the better suppliers of BP stuff.
http://www.trackofthewolf.com/Index.aspxThere are several others, look around. Google is your friend!
After looking at your vid, cutting off the breach plug area of the barrel was my first thought. Just eliminate that possible, and dangerous, problem area.
After that it's just a matter of picking a plug: one tall enough to be just higher then the top flat so you can file it down, and broad and long enough to cover the old tang inlet area. Then all you have to do is tap & thread the breach end of the barrel to match the plug thread pitch and fit the plug. No problemo!
Well, actually You would also have to inlet the tang, locate, tap and thread the powder drum and locate, tap and thread the nipple hole.
It's in those areas that I can forsee possible problems...
Mostly locating the center point of the powder drum on the barrel.
It would seem to me, and I'm at work so I can't check my old CVA Mtn. Rifle, that the positioning of the drum so that it's relationship to the hammer is correct might take some careful consideration. Remember, the CVA's plug is drilled out in the center and the center line of the drum is located as to end up in that hole in the plug.
Generally, the plug should have 1/2 inch (+/-) of thread inside the barrel. The center of the flash hole is generally 1/8 inch (+/-) ahead of that. Naturally the lock has to be positioned so that the nose of the hammer is centered on the nipple in the drum (after you locate, drill and thread the drum for the nipple).
In your case the lock is already positioned and... it also may have a cutout for the drum. If so, the cutout fixes your drum position with relationship to the lock.
Now, if the CVA plug is threaded, say... 3/4 inch into the barrel, and the hole in the plug is 1/4 inch deep, you may not have to do much if any adjustment of the flash hole location outside the nominal 1/2 and 1/8 inch formula. Other then that you are going to have to play with the positioning of the drum, fore and aft, to get it to line up with the hammer nose.
Or, you may have to set the barrel back in the stock to position the drum correctly...
Left and right alignment is done by the positioning of the nipple on the drum.
Like I said, no problemo!
Lemme see here...
Is all that right?
Well, if it ain't it oughta be!
Tell ya what... cummon over to my house and we'll take hacksaw firmly in hand!
Sounds like fun and heck, it ain't my barrel!