I am not a fan of mixing drivers in the same cabinet. The air in the enclosure couples the drivers acoustically, and they must be closely matched to prevent odd and potentially unpleasant, possibly even dangerous (to the drivers) things from happening. In view of that, I suggest trying to find some original drivers.
Failing that, and it's not easy to do, I'd get a set of new drivers, all the same. What kind? Here's where it gets interesting.
The 612S has very little interior volume. Because of that, you have a strong possibility of getting a large peak in the response in the midbass. Minimize that by using low Q drivers. Unless, of course, you like that sound. Either way, you're not likely to get much low bass out of it. It should be loud, though!