+1 ^^^.
-Tapping a torx bit into the hole may have enough grip to turn it,
-Bolt/Screw extractors
Screw Extractors - Ace Hardware
-Remove the other pully bolt and try to rotate the cage halves in opposite directions which may loosen the bolt a bit,
-Drill off the head of the bolt and you'll have a long enough stud left over to grab with pliers.
EV-RD-M663-3043A.xls (shimano.com) Gives you the part number (#10 or 11 in the diagram) to do a search for a replacement or go to your local bike shop.