Crumbs... memories of gazing through shop windows come flooding back!
Roger Squire Ltd., made disco boxes in the 70s... here's an
old catalogue, complete with obligatory dolly bird draped over the speakers ;-)
The speaker does look to be circa mid 70s (it's a ferrite), has a relatively small voice coil diameter and probably handles only 100W. Any replacement should be designed for horn loaded use.
I think it's fair to say that single 15" W bins won't provide staggering low frequency reproduction - it helps to stack about 4 together.
It's very unlikely that a modern driver will behave the same as your existing vintage 15". You would really need to put new drivers in both boxes, at which point the box design itself might be a bit of a compromise....
I suspect that a modern box would perform better ;-)
Simon