There are a couple of options. First off, I'd liberally soak the cover and the screw with PB Blaster. It will break the bond that's developed between the steel screw and the cast aluminum cover caused by electrolysis. Let it soak for a day at keast, applying PB more than once over that period.
Then you have a couple of options. You can either take a small chisel and a small hammer and back the screw out or you can carefully drillinto the center of the screw about 1/4" deep and use a Spline Drive extractor to remove it. The Spline Drive extractors have a left hand spline helix that causes them to tighten while turning left. Hanson Tool sells the Spline Drive screw extractors.
After you get it out, I'd suggest running a metric tap (preferrably plug variety) down each tapped hole and then coat the new screw threads with Never Sieze before installing.
If you were here, I'd take it out for you.
I also suggest changing from the Reed&Prince cross slotted head screw to a flat head-countersunk allen (hex key) machine screw. They are easier to remove and the chance of stripping them are pretty slim.
Hope that helps. The buzz words are carefully and deliberately.