I'd get chamber sleeves 14 ga to 20 ga made up with a new extractor. You can glue them in and heat them up to remove later.
I have a single shot German 16x2 1/2", I glued a Gaugemate 16-28 into place, built up the extractor, and use it for shooting magpies around the farm. I am looking at a nice 10ga English hammer gun, I plan on glueing 12 ga Gaugemates in those chambers too.
Mike