Hi, Bob. I think you run a great risk of this vintage water transfer decal coming apart at the new "cracks" you mentioned, when you wet it prior to application. I say this based on some prior experience.
IF you decide to try installing it, I'd suggest you take a decent photo of it, BEFORE the attempt. That way, this small bit of "history" won't be lost forever, if your installation attempt goes south. There are guys who can make you a new, digital image water transfer decal based on a good photo. And, that might be the best way to go, rather than attempt to use this old, original decal.
If you do decide to use the original, I would not try to flatten it in advance. The less handling, the better. No reason to potentially create more cracks to the surface by trying to flatten it when dry. Also, I feel that warm water and a very small amount of liquid dish soap, is definitely the way to wet and transfer this decal. I believe that's what Mike Slama, the water transfer decal guy, suggests, too. Then, you'll also want to apply some of that soapy warn water to the surface you're putting the decal on, and slowly SLIDE the wet decal onto the surface.
Please keep us posted, and best of luck to you on this. John
Last edited by Jolly-John; Sun Jan 29 2017 09:51 AM.