Hi Leo,
Presumably this kit is long OOP. Standard practice with yellowed decals is to put them in a clear plastic bag and tape the bag to the inside of a window which receives direct sun. A week or two and it will fade the yellowing back to white.
You may assume that being that old, the decals may crack when put in water - A coat of Microscale's Liquid Decal film will prevent this. If you can't source it locally, it's avaliable from Frontline Hobbies in Newcastle.