GIMP :: How To Repair Water Spots On A Photo

I'm new to Gimp - I've had it installed on my computer for a while now but haven't tried to use it because it's got so many features it was just - well, intimidating. But now I've started a big scanning project involving a lot of old family photos, and as you can imagine they're not all in good shape. So I have some repairs to do.

The first one I scanned turned out pretty nice, but the second one is challenging me. It dates from about the 1920s and looks kind of sepia-toned but I had to scan it in gray shades because of a kind of moire pattern at the bottom (it's mounted on a piece of thick cardboard which is bowed and I had trouble getting it to lay flat on the scanner bed, so maybe that's why). The black-and-white image that resulted actually looks better than the sepia, but there are a lot of water spots in the upper left corner. Some of them are really large and in many different shades of gray, which is making it almost impossible for me to fix them using the cloning tool. I thought about just copying and pasting from a better section, but all those different gray shades mean that didn't work very well either. I can crop some of them out, but not all, and besides, the cropping changes the composition of the photo, and not in a good way.

Obviously I don't have great skills with this software (although I'm thinking that might be different by the time I get this project finished ). FYI, the version I have is 2.6.11, which is probably old; I could update if necessary. And it's running in Windows XP SP3.

See the image.