Well, first I would make a good color copy of what is left. That is so if what you try does not work, you have a copy to make your own decals with a color copier or printer. Then, get some Microscale Decal Film, cover one or two of the images with a couple of coats. One coat should be sufficient, but if they disentegrate, put two coats on. Let dry over night. Cut out, soak and remove the decal as usual, be very careful with it, handle as little as possible, see if it stays together. If so, coat the rest of the decals and go for it. If that does not work, try spraying a section of the sheet with several light coats of Krylon clear spray. Again, try the decal, if it works, spray rest of sheet with Krylon. If neither of these work, unless someone else has a good suggestion, go with "Plan C" and make some decals from your copied decal sheet.
Usually, Micro Decal Film will fix the problem, I use it most of the time to fix old, disentegrating, decals. Krylon works good too, good for large sheets that need fixing. But, sometimes, they are too far gone and you have to make your own.