If all that is wrong is something to do with the screws, this will not be hard. If it is the threads on the valve body that are bad.....take the SCREW to a hardware store, and get a TAP the same size. This will be used to "refresh" the threads. If the SCREW is bad, just get new screws. A plumbing supply, or a major industrial hardware store like Marshall's Industrial, Pell Mell fasteners, etc. would have screws that would work.
If the valve body is installed in the wrong postion, or crooked, or too far back.....that is a bigger problem.