yeah I have a theory on why these crack. I'm thinking that the coating on the plastic gets scratched by the (rusted) metal cover. It takes years, but the plastic underneath is vulnerable to attack from low levels of gas fumes or just stress cracking, maybe from slight movement under pressure. Seems every one I see that's cracked follows the pattern of scratched areas leaking.
Anyway, I'd use a new metal ring or at least clean it up and maybe paint it. I actually created a "gasket" layer between the ring and filter body. Basically just a sheet of polyethylene, like a coffee can lid. Not a big deal - should last several years even if you just put in in rusty, I'd guess.