Well, once it's been mixed down, it's tough to get rid of stuff (other than center-channel items like vocals, etc...). You'll probably want to knock some of the mids and highs out, but you're gonna lose some of the vocals, top end of guitars (especially the pluck of acoustics and the scream of leads), and the cymbals/hihats...
Do you use a DAW (computer-based recording program)? You may wish to simply find some of the phrases that don't contain keyboards and repeat them and arrange them as close to the song as possible, that might work. As far as the MD-8 is concerned (is this what you're using, or are you using a consumer-based minidisc player?), you'll need to either A-B repeat the "keyboardless" parts of the song, or find an EQ setting which suits you.