Twitter user stacksmashing has hacked Nintendos Game & Watch to get alternate games running on the device. They started with Doom, but have since succeeded in getting other retro classics like The Legend of Zelda, Super Mario Bros. 3, and even one of the original Game Boy Pokmonmons to play on the handheld. Switching out ROM files requires physically opening the device and using custom hardware to reflash its memory chips, which of course requires a certain level of skill and proficiency.”]
Source: https://gizmodo.com/the-hacked-nintendo-game-watch-now-plays-pokemon-and-1845755381