How to legally go about writing a story based on a 20 year old forgotten video game?

I’ve been developing a story based on a game I used to play when I was a kid. Published by Atari, absolutely loved it, but the game ended on an incomplete note and I wanted to put my own spin on it. The plot and world building is about the same in my story, but the characters are completely different personality and appearance wise.

If I ever get to the publishing stage, say a comic or a novel, how do I go about noting the inspiration for the story? Do I still credit the original source material despite it being so unknown?