In addition to a credit line, I'd even put it's own clause into the FAQ. You can explain that it's a concept that you became attached to at XGame, and due to unfortunate and unknown circumstances, the game quickly closed. This would be a nice place to include a line about whether or not you'll be open to receiving characters from the old incarnation of the game, or adapted versions. I'm sure that other people who played would be happy to see that they can carry out their lines, even if it's been quite sometime.
If the original mod journal is still up, you could always email the mods at the registration email, or leave a note in the dropbox. Just politely state that you are bringing the game back to life at X journal, and that you will credit. Chances are that they registered with a mod specific email, that they no longer check- but in the future, if you are ever contacted by them, you can clearly pinpoint when you contacted them with your intent to revamp the game.
I think that you're going about this the right way, and wish you luck with getting it off the ground.