The drivers, crossovers and cabinets are all designed to work with each other. It would be difficult to replace any of the components and get equal or better sound/synergy.
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Is there anything wrong with the drivers (like foam surrounds that have seen better days)?
There are possibly DIY re-foam kits available for your mid & low drivers if this is the case.
Otherwise, I agree with Jd.
Been playing around with 3-4 sets of vintage single & coaxial drivers (various cabinets) over the past 2 years and it's not an easy task to get things right by ear. This is with but one driver per cabinet (the coaxials just use a single tweeter cap).
Recently tried cabinet design freeware, but only plugged in the specs for a single full-range driver (not certain if it does multi driver systems). Just learning the ropes @ this point and have yet to construct/test the projected cabinets.
Still if you want to take a crack @ it check out the Pi and Madisound forums for ideas (they discuss multi driver system projects).
If you plan to get some other speakers to listen to, you can use these old ones to experement with. It can be interesting and relatively inexpensive, but will take time.
Keep one speaker stock, and work on the other one. This cuts costs in half, and gives you something to compare against so you know if things are getting better or worse. If you end up with something that you like, then fix the other speaker the same way.
Buy Vance Dickason's Loudspeaker Design Cookbook.