That's a tough fix. The plug should seat on the inside against the barrel and on the outside . If you shim it at the back you leave the threads exposed inside to black powder corrosion when you wash it down to clean it.Now the question is why was the plug turned? was it leaking? we don't know, but can assume it either leaked or had a badly stuck dry ball in it.
My first thing to do if it was mine would be remove the plug and see what the threads are like.and probably take a tap to them to clean them up.
Now we need to get the plug to seat on the inside and have the lug in position as well as have the plug seat against the outside of the barrel .
If we knew the material the plug is made from the end could be Tig welded to make it longer , then squared up so it will seat on the inside . This will leave a gap on the ouside so now it is a matter of filing down the plug a hair at a time until it seats on both ends with the lug in position and the engraving lined up.
Lots of trial and error,but possible if it can be built up without destroying it. You could also braze the plug face on the inside to build it up. Looking at the photos again those fancy nubs look like pressure relief plugs, so welding / Brazing probably isn't an option because of the heat needed. I think you have a wall hanger.
Before you give up, pull the plug and see just what you have, then make an informed decision.
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