Mr DTM, 6 db crossovers are ok if and only if you have very rare , high quality , wide frequency speakers.. which is usually not the case.. most of those speakers start distortion long before their specified max output because of unfiltered sound signal..
You can't really generalise that much. As Mark said, drive units, cabinet and crossover are part of a whole. It's a bit like pistons, rods and crank on an engine. Change one without considering how they fit into the whole design and it just won't work.
Some designs can work with a gentle crossover, some need more aggressive filtering.. If the end result works, then the job is done, and how it's achieved isn't really an issue, but choosing drivers that avoid too complex a crossover will increase your chances of success.
I'm not familiar with these speakers. Some older designs still sound great today, others really show their age. Where these lie will determine whether it's worth getting them re-coned or not, IMHO, but you'll be very lucky to find modern drivers that work in the cabinet without a complete redesign and a good bit of experimentation.
Also consider that the undamaged drive units might not be performing at their best after 50 years...