It's also worth pointing to Kaze's videos for what dedication can bring even without the original source code. You'll find among other things recreations of later Mario games, or even games like Portal (and mechanics!) implemented in SM64. https://youtube.com/user/KazeBG0/videos
I went to college with him, and I genuinely had no idea that it was him until several years afterwards. I definitely wish I had realized back when I had the chance to pick his brain about this sort of thing!
His explanations are mind-blowing as well and fun to watch :)