I have a set or two of those OEM boxes, but never have replaced the speakers. Custom-made boxes and bass tubes were what everybody did back then.
I'm guessing a 5' or 5 1/4" would fit there. A decent front-loading aftermarket speaker should work fine.
If it were me, I'd get the hole diameter, and the mounting holes center-to-center, and then beat up the aftermarket websites (like Crutchfield) looking for dimensional drawings of likely speaker candidates. Slight mods to the box would be easy (trimming the hole, new mounting holes), but there's also a lot to said for keeping the OEM boxes relatively intact. A lot of that old OEM sound equipment got trashed.
I would love to know what you figure out on this.