centv4201 Hello! There are a lot of ways to get a javascript game to work in Monogtari, but I need a bit more information to know which of those right ways would work best for you!
One way that works, in general, for me, is to make the javascript game its own thing in the index.html file, and show/hide it using Javascript. A simple function in Monogatari to make the other javascript game visible while also hiding monogatari works well enough, and then do the reverse in the game.
If you have more specific needs, feel free to ask!