Is that a heli-coil already in there? Man.. I guess adding material and then tapping is an option, but seems pretty damn hard considering the location and accuracy needed to line up the hole with the parts to be fastened. Keep in mind I am not looking at the thing in person, but if it was mine, I would try removing any loose bits, the existing heli-coil (if there), and cleaning it out real good with alcohol and a brass brush and see if a heli-coil can bite enough at the end of the hole. If you can torque it down to close to spec without it spinning, I would then remove, clean again with alcohol real good, rough up the surrounding area, create a form to recreate the broken block portion, then use JB weld "steel" high temp epoxy on the installed bolt, helix-coil, and block to set it in place with the tensioner installed (but no belt until the epoxy is totally set). I don't know- as I type this it seems sketch. Hopefully others have some way better ideas. Maybe just the heli-coil could be epoxied into the hole before the tensioner is installed. I don't know..