fettler
Senior Retro Guru
A good chance it will just wind out, the head probably broke off by over tightening rather than being corroded in. Obviously you can't use an allen key now but just pressing something sharp like 2 small nails taped together and pressed onto the remnants can be enough to wind it out. A tiny blob of superglue on the end of a matchstick can bond and then be used to wind it out. I'd always try the easy way before bringing in the heavy tools.