Re: Broken brass/bronze fitting. I'm stuck...
Have you tried a screw/bolt extractor? Extractors have reverse, tapered threads and are no larger than a tap, so it might be small enough to fit in your tight working area. Normally you would drill a hole in the screw/bolt and then thread the extractor into it. Since you already have a hole in the remnant, you are already half way there. Pick the extractor that's the right size for the hole you already have.
If the material is brass, it may be a little soft for an extractor to work but if it is bronze, it might. If I were in your shoes, I'd soak the area with PBlaster for a while in the hope that it soaks into the bimetallic corrosion area, then try the extractor.
If the extractor does not work you can then do the drill out routine others mentioned earlier.
Good luck, Rob!
T. P. Donnelly
S/V Tranquility Base
1984 Islander 30 Bahama