As others have noted, once a recording has been mixed, processed and released to the public, it is what it is. What you are talking about is unscrambling scrambled eggs. Good luck.
The other problem is that if you do stumble across a piece of equipment that, to your taste, improves the sound quality of bad recordings, it is also going to mess with what you hear on good recordings.
Pick your poison
The other problem is that if you do stumble across a piece of equipment that, to your taste, improves the sound quality of bad recordings, it is also going to mess with what you hear on good recordings.
Pick your poison