Try Ilford's Ortho Plus if you have access to a 4x5 camera and plan on copying them. Next to Kodak's discontinued Pro Copy, Ortho + is about the best copy film there is, if you don't need any red sensitivity. The best way to do it, is not to use a yellow filter, but rather a blue filter--or an ortho film. The blue sensivity helps extract detail out of the faded image. You use your processing to shift the contrast around as needed. I copy faded images all the time at the museum where I work, and we use either Ortho + or TMX in 4x5. We use wratten 25 and 47B and some green filters, but only the yellow filters for copying newspaper stock basically. You have to look at the stain and decide what you can do, but most of the time, you're dealing with a faded yellow image, and the ortho film can work miracles practically on it. If the image has alot of inherent reddish/brown tones--like old POP prints--we use TMX and push it...same concept as regular shooting--exposure gives you density/dev for contrast. Orhto + only comes in 4x5 or 8x10, but for yellowed prints, it can't be beat. fwiw, alot of folks use a yellow (or amber) filter when they dupe old negs if the base is stained when making the interpositives. They use a pan film for this, and then an ortho copy film for the dupe neg.

KT