The long way to restore them is difficult, but is the only way to really make them right. Scan them at high resolution, load them into a photo editing program, mask the images and drop out the yellow background. If some images aren't clear enough you may have to redraw them. Print them on new inkjet clear or white background decal paper. White background paper is necessary for any images that have white in them. Inkjet ink is water soluble so the must be coated with a decal sealant like Microsol decal coating.
If I'm developing a habit to scan all new decals. You never know when you screw something up and having a backup can be wonderful.
When printing an all-white images on white background paper you'll need to outline the image with a black hairline to provide a guide to cut the decal out of the sheet. It's why commercial decals are printed on a blue background so you can see the images.
Like I said, this is a long and skilled process requiring some graphic software expertise, but it really works. I've been the dedicated decal customized for our model club.