Sheared bolts

I recently tried to remove my AAV to check its function, and managed to shear one of the screws. Then discovered that someone had previously sheared the other screw.

AAV working ok, but how do I secure it. No access round the back?

If you can get an angle drill (90) and it is reversible, then you can try left handed drill bits, but I would soak them in pb-blaster....first.