I don't have a copy of the Fiend Folio, but I've seen it. I was thinking about Gods today, well specifically God stats.
My first thought was HPs, gather together every d20 that you have at your table (including all your player's 20s as well). Roll em. That's the God's hit points.
Spells, see that book you are using for the game, the God knows all those spells. Plus a few extra Godly spells.
Immunity? Sure let's go with almost everything. Alright fine, give em one weakness.
Damage? Whether its a hammer, lighting bolt, a dark curse or a large bicep, gather up all your D6s. Even the ones in all your board games. Roll the whole set twice (or just multiply by 2).
What can hit em? Spells possibly, and +5 weapons.
What happens if the God actually get's killed? There's a slim chance, but maybe they are sent into the underworld, maybe they recreate themselves in space and rule another planet, maybe they are reincarnated into a far eviler terrible version of themselves? Who knows.