My 2000 medal is discoloured, and has a green blob of corrosion, but I always put it down to the amount of sweat I got on it wearing it for 3-4 hours after the race, which I never wiped off. I haven't tried to clean it, as I suspect it's painted rather than all-the-way-through metal, but you could try someting no too strong, like washing up liquid solution, and see if it works. I doubt you could get a replacement, but it's worth a try.