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.
Thanks for the advice Flip. I was able to use my hammer to bang in my largest Philips head screw driver and it got enough bite for me to remove it. I did put in a liberal amount of PB Blaster last night so I am sure that helped with it as well.
Where do you get the hex key screw from? Is that something Ace, Lowes or Home Depot will carry? Also good advice on the tap and Never Seize, I have both and will do what you suggest.
Any fastener supplier in your area will have them. By the piece or by the box. Enco also has them www.use-enco.com I buy most of my tooling from them problem is, their minimum quantity is 100 units but you can buy a hundred of them for about 5 bucks and that's in stainless. The best thing about switching to a broached hex drive is not only positive engagement but the fasteners are at least Grade 5 alloy which is much stronger than the OEM unmarked stuff.
They are on page 641 of the master catalog.
Ace might have them. Lowes or any box store most likely won't. There is an Ace near here and when I need an odd fastener, especially a metric or ISO one, they have it, at a price of course.
Have you had any trouble with PB Blaster and plastic, I know it dissolves styrofoam on contact but I'm not sure how it reacts with plastic. There's not a ton of plastic on the bike but the gauge housing and clear face covers are, hate to see a bunch of fog spots all over the place from splatter.