If the Allen head is rounded on the inside .... you have two choices fark around using an easy out, or mig weld a bolt onto it ( head face out )... two things happen here, heat is applied to allen thread to loosen it up and you are able to hold turn it.
This may take a couple of attempts depending on your welding skills
the same thing is down when you break a head off a bolt ....put a washer under a nut, mig the centre ( fill with weld ) let it cool down ... 90 % of the time it will come out.