A good way for removing and preserving these stickers is patience... and a hobby knife.
Soak the hobby knife in white spirit (mineral spirit) and little by little try to lift them by applying a tiny bit of the liquid under the decal to dissolve the glue. Decals from this era must be chemical resistant. However I would try to see if white spirit lifts anything other than glue first (colour for example). If all the glue is dissolved, 3M adhesive transfer tape is your friend.

Another good way: After I found the right font of these stickers (helvetica) I recently prefer to make a nice clean version in photoshop and print them in vinyl decal paper. After that I spray them with acrylic clear, leave them for one day, and have new-looking decals the next day to put in my machine. Noone can tell the difference.

Btw, I'm a purist, I like things kept original, like you