Thanks all for the suggestions!! Might have to go with the easiest one as time is short. The isolated bass line is just a youtube version, so there are artifacts etc. on the mp3, but I think it will do if I can get the tempos match up. Might be able to do some clip editing on the parts of the bass track that have a lot a lot of background noise... not sure what they used to extract it, but its OK for my purposes. Probably used software similar to isolating vocals for a karaoke track??
A thought crossed my mind just now--- maybe I could find the Guitar Hero .mogg files and convert them to midi...
With regards to changing a Mac bassline, I think what Chuck was referring to was the "midi" bass line. He would go in and change it to the Mac version, not the other way. The midi bass line is pretty bad.
Thanks again for your responses and timely assistance!