Has it snapped flush with the engine casing?
If not remove gearbox and try grips on it.
The threads on engine side are a Coarse thread, try tapping it round with a pointed punch with a bit heat on the alloy casing should come out.
Or drill a small hole in it and hammer a screwdriver into hole and try turning it out, with being coarse thread they usually come out when you do not want them too.