Look here: .
https://www.youtube.com/watch?v=i-8G5AaVD3o
At 9:32 Bob Fowler explains how to loosen a pushed in stop motion bushing. I don't think one needs the tool he uses. A $10 plumber's faucet remover might do, a few small taps with a hammer all around the bushing would also be something I would try if it were me. He also welcomes emails with questions about machine repairs. Use "My Worst Nightmare" in the subject line.
Also, if you haven't removed the hook assembly, I would certainly do that to look at whether a piece of a broken needle or a entire needle has slipped inside the assembly.