As AG says (#6 above), in an ideal world you'd want drivers matched to the originals so that they interact with the box in the way that the designer intended.
However, we don't live in an ideal world - we live in a world where you've been given a pair of boxes and want to make a gig happen (it's ok, that's the world I live in, too). With this in mind, I wouldn't worry too much about what drivers you fit; in reality they cant be too far wrong. The most important factor is the free-air resonance - this should match that of the cabinet for what they call a "maximum power alignment". The problem is that we don't know the resonance of the cabinet, but looking at the data sheet (figure 8, assuming that's measured with the driver fitted in a cabinet like yours) it's around 50Hz. I think that many 15" drivers may be a bit below this, but not too far.
Personally I'd prefer an 8-ohm driver to a 4, as many amplifiers rated for 4-ohm seem to struggle rather more than I'm happy with at this impedance. This also gives you the option of slaving another passive 8-ohm box from each active one in future (in case anyone gives you another pair of cabs!). Also bear in mind that many of the amplifier modules that can be fitted to subs are run in a bridged configuration, which effectively halves the impedance, so many of them are only rated for 8-ohm in this context.
Essentially I'd look for a pair of used 15's in good condition and fit them. Good luck - don't forget to post pictures of the finished boxers!