I replaced the battery in my 990 with an Optima shortly after I got the tractor. Not because corrosion had started, but to prevent the problem before it happened. I'm not sure of the exact Optima model - it was whatever Wall Mart was selling at the time. It probably doesn't even matter too much anyway. There is lots of room up front for whatever battery you might want to put in there, and in my case at least, the cables are long enough to reach to whatever side they need to. The only "modification" I had to make was putting a strip of flat bar behind the battery. The rounded corners of the Optima don't mate will with the little corner braces designed for a square battery and the strip of metal gives something for the whole back of the battery to sit against.
Another reason I put an Optima in was because they are supposed to handle deep discharge without damage better than most batteries. The alternator on the 990 is pretty wimpy and I added a bunch of lights, so I didn't want to be killing lesser batteries by draining them too much.
Good Luck!
