Does the tenon screw into the shank or stem? Or does it push into one or the other ? If it pushes in try measuring the opening and you should be able to order a new bit. If it screws in I dunno..

. if it is a push tenon you can remove the old one by carefully screwing a wood screw into it and pulling it out, then you could measure the diameter of the tenon.If you pick up a bit that the tenon is too big, I would use sand paper to reduce it until it fit.If it screws in you might try using a small flat blade screw driver to twist it out,and maybe superglue or gorilla glue to repair...?