If all you have are mixdowns and have no original individual instrument tracks, then you are already past the mixing stage and there's no turning back. There is no way to "un-mix" a mixdown.
It is possible to do some mastering - defined in this case as "doing what can be done to make the mixdown sound better than it currently does", but based upon your description the quality of the results will probably be quite limited for two reasons:
First, if the song(s) do need re-mixing like you imply, mastering can't help that. The best that could be done in that regard would be to perhaps apply some EQ to try and emphasize one instrument or de-emphasize another, but more times than not, that won't sound all tat good.
Second, MP3s are useless for engineering; they should only be used once a song exits the engineering environment, and even then only when necessary (when used on an MP3 player or for Internet distribution.) Your MP3s can be converted back to WAVs for the mastering process, but they'll still be missing all the sound that was lost when they were originally converted to MP3, so the WAVs would be of an inferior grade to start with.
The fact that you say you use C.E.P. implies to me that at one time at least you had the original WAV tracks, or at least some of them. If so, those are what you want to work with. But if when you say all you have are the stereo MP3s, then the above description applies.
HTH,
G.
