It should be possible to build a pair of subs using the original enclosures, or in new enclosures that replicate the internal air volume of the original enclosures, using the original passive radiators. If there are any concerns about the passive radiators, you should probably have them re-foamed; Parts Express has a speaker repair center that does a good job.
I have four VMPS 12" WCF midbass drivers (single magnet, solid pole piece) without phase plugs that I bought second-hand, and I have always been a fan of the unconventional VMPS bass loading principle. Dayton Audio has a 12" passive radiator that can operate down-firing, though not with any additional mass, and I have thought about buying a couple to build a stereo pair of subs with the WCF woofers I have. The only thing potentially off-putting is the size of the box required.