So the threaded neck is still stuck in the head, but the hex headed shoulder broke off? Wow! I have never heard of that.
Perhaps welding a large-ish bolt onto the stub will allow you to extract it that way? Make sure to disconnect the electronic ignition module ENTIRELY from the wiring harness and ground BEFORE electric arc welding of any sort! I good, deep, solid bead all the way around should suffice, and let it cool fully. Also (of course) apply penetrant lube just before applying torque.