If the devs released the source code, it could be possible to add yt-dlp as a dependency, so that every time YouTube changes their interface, the good folks who work on that project will automatically fix this one, too. I remember playing this and it was awesome, I'd love to play it again.
Viewing post in Retro YouTube Simulator comments
In the meantime, I figured out a way to sort of hijack the search request that it does and make it play your own video. I wrote a little tutorial for it here: https://gist.github.com/jericjan/b40b3146c1a485c3d1f37a1bf22a6836
EDIT: Added a second method that directly mods the game itself, making it a simple local video player instead.