Incoming in v2: gender-dependent storyline!
[video=youtube;6mSEx6OxzmQ]
Roughly 80 bytes of ASM code and a duplicate of the a/ folder (named z/ instead to ensure the Files ID are kept in the same order) to make this possible.
You can see there what a Gen IV Pokémon game running at 60 FPS looks like (be sure to watch the vid at 720p60 then).
And also Kris, to make a true tribute to Pokémon Crystal.
Basically if you play as a boy, Feebas and Milotic are unavailable in the wild, whereas if you play as the girl, Magikarp and Gyarados instead are the ones nowhere to be found.
In this hack this is not an insurmountable problem since the Magikarp/Gyarados family (who are male only) and the Feebas/Milotic family (who are female only) are considered the same species, which means you can get the missing counterpart by breeding with Ditto.