First of all, yellow or brown is not a sign of water, it's caused by acid in the backing paper staining the carrier film. And it can be fixed. Expose the decals to direct sunlight, usually by taping them up in a window facing south, for a month. Just watch to make sure its not one that gets condensation. I prefer the living room. Try to kind of time it so that you use them in the next month or so after the initial 3-4 weeks it takes to bleach them. You'll be pleasantly surprised.
Scan them.
Then, run a test with a piece from the sheet you can live without and try to overcome the next test; breakup.
If you have breakup, which you probably will not, it gets hard.
If it's bad, you need to bind them with a couple of possibilities. Buy Microscale Decal solution and paint it on, wait a week and try it. I prefer spray with a laquer like dullcote or Glosscote from Testors.
Or , put a puddle of Future floor polish where it goes and pick the dissolved pieces into it w/ a toothpick.
Keep me posted, and I've generally survived these problems.
Bill