It's a very cool hack, highly recommend!
It lets you switch between 4 different characters on the map screen. Each one have unique physics and a special move. Also there is one additional unlockable character.
Bosses also very interesting. Each have unique mechanics. They are way harder then the original, but not super difficult.
What I didn't like is that many surfaces are slippery. It makes the game feel a bit unfair sometimes.
Retroachievements for this game are fine, but I disliked the speedrun and deathless one.
Here is the link to the patch.
I really need to connect my pi Gameboy to Retro achievements