First thing I would do is to try an impact wrench and maybe a little heat, you may just jar it loose. If that doesn't work, since the head was soft enough to strip you may be able to drill the bolt and use a screw extractor, or just drill the head off first and then use extractor if necessary, depends if it is tight in the threads or tight against the head.
When you get the screw out replace all the screws with new Allen bolts.
When you get the screw out replace all the screws with new Allen bolts.
