if you have any torx bits, take the one that's barely too large to fit in the rounded out hole. persuade it into the hole with a hammer (don't wack it TOO hard... remember the cases are aluminum as is the thread the plug goes into)... once the torx bit is seated well into the bolt, try to unthread as normal. the torx bit should have a good "grip" in the hole and the impacts required to drive it into the damaged plug should help unseize it.
then... get an aftermarket replacement plug.