I'm with FB on this one. I would clean and grease the bearing and use it till I knew I'd solved or at least identified the weather/rust problem. Unless of course I couldn't adjust the slop out of it, then I would park the bike until a new bearing could be had. Buying through a bearing house makes for very inexpensive bearings these days. Someone mentioned prior about swapping the upper with the lower to get more life out of them, all but one of my bikes uses different sized bearings upper and lower excluding the free ball ones.