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Mastered Super Mario Bros Mini!
(cdn.masto.host)
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
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".