mmsailor;
I had to replace the mast light on my H260, water came in and the lens socket showed badly oxidation. I put a combination of anchor and tri-color navy light instead, replaced both bulbs (anchor 10W, tri-color 25W) with LED. Found for the anchor light even one with photo sensor, which goes on and off on its own, quite nice when anchored, go out and come back when dark, the light would go on and off when sleeping in. Well, I am a playful guy.
Regarding your missing screw, when you can take the fixture to Lowes, you may find a fitting SS screw right there. If you can't take it off, I would recommend to bring some small screws to the boat and try to fit one.