Now that is a proper mastering question!
The easy answer is to finish the rest of the project and then hand them over to a mastering engineer with a good pair of ears.
Unfortunately the more useful answer is probably also the more difficult one:
First question, do you still have the multitrack of the original track or are you working off the stereo mix?
If you do have the multi, then have a look at how the vocals sit in the mix, and how powerful do the drums sound?
Is the amount of reverb/'space' in the original comparable to the new stuff?
If you're on the stereo, this is more tricky. If there isn't enough reverb on the old track, then add something similar to what you've put on the new stuff. BUT, if there's too much on a stereo mix, that's hard to get rid of. Either live with it or tweak the new stuff to match :/
The other thing to consider is the dynamics of both old and new. Firstly, does the old one sound as 'loud' as the new stuff? Maybe you need to bring up the level with some soft compression to match the new.
If all else fails, maybe consider something along the lines of a short interlude or change of scene. Or maybe even a kind of demi-bridge? Although this might not match either the old recording or the newer ones, it might help bridge the gap. Even if the sound changes radically twice, it might sound more deliberate than an old recording shunted on the front of a newer, better mix.
Does that help at all?
