If pliers or tweezers would've worked then the converter should have come out without breaking to begin with.
Maybe a tap or a screw extractor slightly larger than the inside diameter of the remaining bit of the converter. Either one would cut into the converter and allow you to twist and pull the converter.
Screw extractors (taps too for that matter) tend to be tapered so you may have to do some grinding to get to the right diameter.
It must be really stuck in there - dried ink? Try cleaning that out first. Water or pen flush or Koh-I-Noor cleaner. Maybe a run through the ultrasonic cleaner.
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