There have been a few posts on removing/replacing rotten body screws. One trick that's worked for me is simply a Dremmel tool with a cutting disk. Just cut a slot in the rusty head and bingo, you can turn the screw out!! If the head breaks off :evil: forget it and just run a new screw in next to it. Also, as an alternate to stainless screws, try the new coated decking screws, they're great too - available in better sizes, and a lot cheaper.
PS.... since the screws are recessed-head, you'll also cut a slot into whatever the screw holds, but so what? The Dremmel cutting discs are about 1" diameter new.... if this REALLY bothers you, wear the disc down to a smaller diameter before cutting the screwdriver slot. Also, whatever those decking screws are made of and coated with, they're guaranteed for practically forever... I love 'em! Stainless ones aren't as strong either.
Andy