Now that I have - perhaps there is a simpler fix.
Looks like the exhaust port has a stub that is screwed in.
The hole is just behind the threads.
If your stub is - or can be modified to be - longer with no threads for a small portion - it can cover the hole.
Further - if it can go until it bottoms out to the edge I am showing below - it would seal that off?
How about just having a ring welded on the port stub so that you can bottom it out? Can you show a picture of the stub?
This flat here - does the stub screw in far enough to bottom out on it?