That is awesome! I don't know if I can help, but here are a couple of thoughts;
Can you describe the plug a little better? I have having a hard time visualizing exactly what it is in there. It isn't quite the same thing, but when salvaging historic engine blocks to run again after sometimes 50 years in water, you have to get out the internals without srewing up the block. There are a number of almost counter-intuitive methods that might help if I knew what you were up against better.
Also, this is a longshot, but are there significant differences in metallurgy that can be used to advantage? Sometimes a small battery and some saltwater can do more than any machining bit. Weak acids too. they all take time but there are ways to leverage a metals difference.