Jaybird,
I have this same cabinet. Mine is dated 9-12-1968 and has the original matched set of Jensen 12" speakers. This cab was matched with the Sceptre amp head and was designed for guitar, not bass. You could swap out the speakers for modern, high-wattage speakers, but the cab isn't ported as you noted. So, you'd be trying to re-engineer a guitar cabinet for bass with ports, baffles, etc. Unless you know how to design speaker cabinets, I'd recommend you leave this fairly original, vintage piece alone. You could likely sell the cabinet for nearly what you paid for the entire rig. Put that money towards a modern bass cabinet!
Ryan