I know what some of you are thinking. "Why not just save it as an MIDI?"
I hope nobody was thinking that.
Unfortunately G-MIDI doesn't have the all of the sounds on that mp3, so it might quite a bit different in MIDI format.
Since this is GB sound, the instruments should be easy, 2 square waves for the lead and a triangle wave for the bass. The only difficult part would be where the notes 'fade' in steps.
Zophar has the ripped soundtrack, and probably some players that can play solo channels, should make it easier to remake. Come to think of it, that site might even have a .gbs to .mid converter.