I have a friend who's a research chemist at Exxon and works in the development of all sorts of rubber & plastics. I asked him this very question about bringing old cowl gasket material back to life. He said that once the plasticizers have dried out & left the material, there's not much you can do to bring them back. Newer polymers have much longer lifespans, but once the old stuff is shot, you can pretty much forget about it.
That's what he told me, anyway....and he's forgotten more than I'll ever know!