Hello!
Thanks for your compliment and your interest in my hack.
To modify this hack, you should not hex edit the rom.
Instead, clone (download a copy of) the source code:
https://github.com/dannye/pokered-gen-II
Then modify the source code and assemble the rom using our Gameboy assembler:
https://github.com/rednex/rgbds
The great advantage of using our source code instead of hex editing is that you don't have to worry at all about free space and pointer values.
The assembler arranges code and data to manage free space, and it calculates and resolves pointers for you.
This way, you can just focus your time on modding the game instead of wasting time on "clean up" and "busy work".
In your case, if you want to replace songs, just take song files from here:
https://github.com/froggestspirit/CrystalComplete/tree/master/audio/music
and put them into this folder:
https://github.com/dannye/pokered-gen-II/tree/music/crysaudio/music
Then you have to modify how songs are used in game by editing things like constants/music_constants.asm, crysaudio/red_pointers.asm, and data/map_songs.asm etc.