Any ideas as to proper drivers Zilch ???
The problem I'm having is the height.
Notwithstanding what may have originally been in there, it looks like centerline of the existing cutout is at ~24", too low for a coax, unless one listens with their knees. Seated in a chair, my listening height is 48". On the couch, it's somewhat lower, but I'd have to be on the floor and slouching to be on axis.
If I put a UHF driver up in the corner, i.e., D130 extended-range woofer and 075 or 077, I don't think it'd be high enough still, because those beam to 30° or less. I'd have to use a wider vertical dispersion horn, or some scheme to aim the HF driver upward toward the listening position, both requiring a reconfigured and/or tilted baffle. The vertically asymmetric MantaRay horn in Altec Model 14 (a clever idea, I think; wonder why it didn't catch on,

) might be an option, but I've never even seen one of those, let alone heard or measured what they do.
Those are large boxes, ~6 cuft., if my calculation is correct, and could do subwoofing with minimal effort, which is why I pop in with that option. What drivers were "proper" when those were made is largely irrelevant with respect to loading them for good sound according to current listening standards....