Stuart,
You can try taping them to the inside of a sunny window, facing out. This
may bleach out the yellow. However, if done too long, other colors may also be bleached.
Another problem is the carrier. It may have decayed. There are two things that (AFAIK) have been used to repair this: clear coats (not acrylic, although Future might work if you let it cure for a week or two) and liquid decal film. The latter is available from some aftermarket decal manufacturers. I saw some at Bare-Metal, but there are others.
But regardless of the above, it would not hurt to start looking for aftermarket replacements.