Snapped off screw on sewer vent cover
First, you can try to douse the area liberally with a penetrant to remove any rust and corrosion then grab the screw with a pair of locking pliers and twist it out.  Rather than pliers, you can try to use a screw extractor bit (easy-out), but that almost looks too small for that to work.

If that fails, the screw can be drilled out and the hole can be rethreaded.  Really what you would be doing is drilling with the proper size bit for that hole and threading with the same sized tap to remove the remaining screw rather than cutting new threads.  If that fails you can simply rethread the hole for the next largest screw.

The main point is you have a few options here, and you really shouldn't need to replace that piece.