A stout wire brush should do the trick, though if it's really rusty you might want to use some steel wool on it too. Brush it off cold to get most of junk off, then put it on the grill and heat it up, then scrape off the rest. I would say that you should be careful not to mess up the stainless chrome finish, but since it's rusted it means it's already gone anyway.
If you don't feel like working it, just buy a new one. They're available everywhere and aren't really all the expensive. I usually replace my grilling surface once a year on my kettle, but I'm kind of anal about a clean grilling and prep area.
22" Weber Performer
Modified Brinkmann Gourmet...the FRANKENSMOKER!