Mdub got ignored...I'm with him. Take the cover back off and enough of the bolt might be revealed to get vice grips on it...soak it with penetrating oil first.
Yeah I seem to get a lot of that around here.
Pretty tight bunch yessirreee...
But hey, with all these great suggestions, I can see where mine might be overlooked.
"You could have the broken bits removed through EDM"
"Get a reverse twist drill bit "
"small automatic (plunger type) center punches"
"regular flat blade screwdriver"
"changing crystalline structures "
"If you have a welder handy..."
Funny stuff!