As usual, Tubs has many good points.
There are things out there that can clean old decals appropriately, I am not sure how the old decals were constructed, so I’m not sure what those things are, but if you get me that info I can check it out. I would recommend (if you have one) using a decal that is pretty much gone from a restoration/preservation standpoint as a test bed to find out what will damage the decals before you start trying to preserve them.