The music is monophonic, as that's how it is in the game. I can't magically make it stereo. (Well, maybe I could, but...
...you would have to recreate each individual instrument track using the closest approximation to the synthesizer used, even matching the recording techniques to make sure it sounds the same (or close enough). From there you could use your recording to reverse the phase and one by one isolate every single instrument track, and then using the original song as a guide, map each track to either the left or right channel as you deem fit for maximum sonic quality. Basically, it's a method that will give similar results to a reverse vocoder, or as it's more popularly known, a vocal remover.
EDIT: Here's some more about phase inversion:
http://en.wikipedia.org/wiki/Phase_inversionAny good wave editor can do this, I think there are also VSTs. The obivous cons are the time and effort involved in such a process. The other big con is as mentioned, your recording of the solo instrument must be pretty close to 100% spot on Ideally this would mean having the same synthesizers, the same outboard gear, same recording/mastering method, etc because if if one minute detail is off it will throw the whole phase out of whack and you'll still hear the other track (kinda like when a vocal remover can't remove the vocals all the way).