What I’d try, is get yourself some decal sealer.
You can get it with a decal kit by Testors.
Scan your decals into a computer for safety.
Spray the original decals with a new coat of sealer and let it cure a couple of days.
If the decals are yellowed, put them in the window and let the sun fade the yellow out.
(Takes several days to a couple weeks in the sun to do the trick)
If they still won’t work after being resealed, make a new set from the copy you have scanned, using the decal maker kit.
just an idea, but how about some kind of flexible clear coat ? ought to work pretty well. alternatively, at 1/24 scale, masking and respraying isnt a bad idea.
I too would recommend the Microscale decal film. However, as old as your decals are, I would apply a minimum of 2 coats, possibly as many as 4. Also, have you considered making stencils and painting the insignia and lettering? Might be quicker and possibly look as good or better than 30 year old decals.
Someone on here posted a message a while back that they would print decals from your artwork. Can’t remember who it was now, but I’m sure someone does. If not I’ll search my bookmarks and see if I can find it.
Perhaps you could scan the original decals and then get some new ones printed from the scan. Or, if you don’t have access to a scanner, send him the originals and have him scan them.