The head is probably a bit low-wattage in the first place given its age, so I imagine that running numbers for anything other than low end roll-off is basically a non-issue (if two modern fifteen inch drivers can't handle 50-75 watts apiece at peaks even in slightly sub-optimal environments, I can't imagine they'd be selling them very long). I honestly imagine you can't go too wrong depending on the tone you want. But either way, doing some quick measurements and figuring out what you need first with some quick math or software running, or calling usspeaker as two fingers mentioned really can only do you some good. If I recall the guys at eminence are pretty open to calls as well.
Heck, I'd have half a mind to cover the port and grab a couple of eminence Alphas, or a similarly cheap driver with 1.5" coil, after looking into the internal volume depending on the wattage of the head. Sure, there are heavier duty drivers and some really efficient Neos with higher power handling - but at some point you're just spending money on either a heavier ceramic driver or a neo driver with power handling that you'll never actually use with a head like that. I mean, if you want a new car, are you going to get a formula 1 racer, or are you going to get something sensible? This is a similar principle, get what you can verify will work, but don't over-build it.
Another potential winner: eminence Deltas. Sure, higher wattage voice coil, and look into the cab and port volume, but it's got good mid-range and high sensitivity, which could make the most of your head's wattage without spending too much money.
