It's a port of the original Super Mario Bros from the NES to the Game Boy Color. The physics feel very nice, and I like how the sprites are adapted.
The port also extends the game. After the 8 original worlds, Quest B adds 8 more with new enemies, mechanics, and even bosses! I had so much fun playing it.
Also, the game is FOSS: https://github.com/Mico27/SuperMarioBrosMini
How can this be FOSS, or for that matter even legal, if Super Mario Bros is a proprietary game?
It's a reimplementation from scratch using GB studio.
But since it uses Mario, it might violate Nintendo IP. It's the case with many hacks. It doesn't make it non-FOSS.
It can't be both a port and a reimplementation from scratch, those two concepts are mutually exclusive.
Yes, I was wrong to call it a "port".