It's licensed under GPL, so you can modify the sources and distribute it freely, but keep in mind GPL enforces you to release sources too. Running New Super Mario Bros isn't hard, I've it running alread. Running Mario Kart / Zelda would be harder, afaik. Anyway, have fun working on it
Oh, and be sure to get the lastest SVN revision, you'll get a more stable version than the lastest release.