The rotor screw is for assembly line production, it doesn't make any difference whether the screw is there, whether the rotor indexes to the hub pattern the same way or anything else. The screw is there to keep the rotor from falling off the hub while the chassis is going down the assembly line before the calipers are put on. You can run without it just fine.

If you just have to repair it, get a left hand drill set. You start with the smallest drill in the set and step drill up until you either get the screw drilled out to where you can pick the remaining threads out, or if you are lucky, the drill hangs up and the screw backs out. Left hand drill set can save you hours of work in many cases.