Howdy!
I've extracted the midi from Slinger Swagger and have tried to convert it using Denote, but I haven't had much luck! For some reason, it only picks up two of the instruments, so it sounds even more bare than the 'midi only' version does; with long sections of silence interspersed with the occasional melody.
It doesn't seem to be picking up all of the midi channels in the file, or perhaps it isn't able to play them simultaneously.
I'm not sure what's going wrong; I've tried removing a bunch of instruments and boiling it down to a very simple midi file, but it still doesn't seem to work.
If you DM me your email address, I'm happy to send you the file - perhaps your luck will be better than mine.
I also looked at how trackers work. A fascinating music approach! I haven't done anything like this before.
Perhaps if I send you the midi file, you could download a basic midi program and try to manually adapt the notes to PICO 8...
Alternatively, you might be able to make your own tunes! This tutorial seems good - maybe it's useful for you:
Good luck!