Don't forget to replace the SPC core with a cycle accurate one, and replace the BRR decoder with one that supports the correct wrapping/truncation behavior! Oh, and maybe port it to Allegro for Windows/macOS/Linux while you're at it.
I feel silly having asked because I managed to figure out what I needed to do to finish my cover.
Here it is if anyone wants to listen. Be sure to use OpenMPT or something based upon it's playback code. The cover was written in the original Impulse Tracker though.