yes, you change the link here https://github.com/cosmo-ray/little-space/blob/little-mage/js/game_level/lvl_01....
It look for music in audio folder. (game_directory/audio/)
The engine is pretty simple, it take map as a global variable, use it as the fist level, and each time there's a .next in a map, there's a next level.
At first(during dev) I was using anime music, but because they're not free, I've change them for free music.
Also as my engine interpret the js code, you don't need to clone the github repo or anything, you can just change the link in the extracted archive. (though I wouldn't mind a pull request :))
map argument are like this:
Map(audio_src,
level_type (0 is generated from the music, 2 is boss and 1 is testing crap),
optional_first_pattern_number,
optional_last_pattern_number);
pattern number are from 1 to 4 (mistake I've made when I've code the engine 6 year ago, it should be 0 to 3) , you could add one by copying a pattern file, add the file in start.json and increase nbPaterns here: https://github.com/cosmo-ray/little-space/blob/little-mage/js/funtions/Normes.js (Norme is another mistake from my early coding, what does norme even mean...).