Drill and use an easy-out. Be careful not to mar the threads in the block when you drill your pilot hole. It shouldn't be too bad - the bolt is large enough. I'm not sure how hard the steel is in the tensioner but from the way it sheared it's probably not case-hardened. Just take things slow.

For the plastic covers - you should be able to find those in a junk yard from any of the other 3.2 or 3.5 motors in Isuzu/Holden/Etc vehicles (also Honda Pilot/Acura MDX).

-- John