Mounting rails rusting away

The underside of the cooking box has permanently mounted rails that accept the bottom tray on my old model 900LP. Both are rusted nearly away. The tray and box are on decent shape. I need to come up with a fix. Any ideas?

There has been many write ups about this The hardest part is getting the screws out. I suggest copious amounts of PBlaster and even carefully applied heat NOT to the screws but to the screw bosses and extreme care and patience. Failing that if the screws break careful used of a drill and new stainless steel self tapping screws with some Never Seize on the threads

Larry is right. Start with some good penatrating oil. If that doesn't work, try heat. If that doesn't work, you probably already twisted one or two off so just keep unscrewing the others until they either come out or snap off. You may have to use a pair of vice grips to get ahold of them or you might even have to grind the heads off.
Then, carefully drill out the old ones. I have found that you can actually drill a new hole just to the front or back of the old one rather than trying to drill out the old screw. If you do it on the same side for both screws, the bracket will still line up just fine.

A new set of brackets will run you about $15-$20 or you can find another grill to cannibalize them from or you can fab up a set for yourself out of aluminum Z stock.
https://www.mcmaster.com/aluminum-z-bars

Thank you both. Good information. I’m on it... soon.

That happened to me on my 17 year old Genesis Silver B. I called Weber, they sent me a new pair. It was a little gnarly getting the old ones off, but a little WD40 and a socket wrench and all good.

Done! Well, for the winter anyway. PBlaster once again did the trick. Old screws backed out, rigged a bracket. Back in business. Thanks!