With B2W2 out, you can use the Kanto Gym leader SSEQ for your game, since all you have to do is change instruments without turning it into a MIDI and it will still keep the loop.Since it's more important and probably a bit easier, I'll do music first.
I remember trying to do something like this, but whenever I tried to overwrite the original gym music with B2W2's Sinnoh gym leader music in crystal tile 2 it kept giving me an error, which I'd assume is because the new track is larger than the original.. Any way the SSEQ files can be shortened without turning them into Midi files?