I can see where you're coming from, Phil, but I'm fairly confident that that Eminence driver in such a little cab will sound like a transistor radio. The problem is, as you have already said, it's a bit too pricey to start experimenting with.
I think the solution has to be to go with the original driver, which is more akin to a car audio driver than anything else. I've heard these combos, and they don't go very low even with the original driver. If it were mine, I'd modify the cab to take a smaller driver. A modern 8 or 10 would work quite well. It's not that hard to do - as long as you have enough space between the front grille and the speaker to take a spacer ring.