[mcain]

The tool that imparts a reversible twisting action to the screw is an impact wrench. It looks like a fat screwdriver and used in motorcycles, etc, all the time to get stuck screws out. Since you hit it with a hammer, it tends to seat the bit into the fastener at the same time it twists. Particularly good on phillips screws.
I have found that McMaster-Carr (www.mcmaster.com) has the best selection of fasteners I have ever seen. They are super fast in shipping orders and don''t seem to mind the order for a dozen screws. SS of various grades, etc. Ship out of Atlanta and other warehouses.
Good luck on the project. I replaced ALL of my 11 opening portlights, and added two new ones, and rebedded all 7 hatches this last winter. Big, messy job. The plastic portlights were so hard to get out that we used a reciprocating saw to cut them in pieces and then to saw through the caulk. If they have used 5200 to bed the portlight, you will find that the stuck fastener is just the beginning of the problem. When re-bedding, use either 3M 101 or 4200, NOT 5200.