Personally, I'd just be sure to make a note somewhere in the mod journal - the FAQ or the rules, maybe - that the concept was inspired by (x) game and you're just borrowing the idea, just to cover the basis and be on the safe side, because at the very least you're giving credit where credit is due. Even if a game has been dead for a long time, mods did put a lot of work into it, and nobody likes to see their ideas being used and not given credit for them.