My goal wasn't to replace the original boss theme, but to move it to the final boss theme and use this as the standard boss fight music.
I retrieved the rock sample from here: https://www.looperman.com/loops/detail/247100/on-my-way-rock-guitar-intro-free-144bpm-trap-electric-guitar-loop I also used the crisis sound font for the drums and some other effects from lmms. Laugh from the scratch sample library.
