Agreed, years ago there were quite a few OEM and aftermarket manufacturers of drivers, most of which had no available data. Radio Shack contracted with a few, there were also companies like Oaktron, Utah and Quam that made inexpensive 18" drivers for these applications.
IME, these were drivers that were intended for boxes to be designed around by using the tune and measure and then retune and remeasure method. This CAN work well for somebody who understands how ported cabinets work, but takes a lot more time, effort and expense. This is exactly how and why TS methods and parameter characterizations came to be.
IMO, the best way to move forward is to look at what drivers are currently available that have extended HF response (for an 18" that is), but it also needs to have good polar pattern as some 18" drivers are abysmal in this regard. THEN, the suitable candidates will need to be modeled in that box (approximation since the flaired port is not directly modeled in most analysis software) to see which candidate works the best. If weight is not an objection, there are some reasonable choices IME.
Also, you might check to see what driver was standard, and then look for a used one. Reconing this era of driver with correct parts is going to be difficult at best, generally the shops will look for parts that fit and build up something that will make noise because the original parts and parameters are unknown. It's possible that this driver was also used in some of the Sunn PA cabinets of the day, Fender's Sunn support site might offer some clues to this. I believe Sunn called (or branded) these drivers under the Magna label.
While good advice I think the chances of finding one are close to zero and if you do find one, chances are it has been setting in a garage or basement and has dry rotted or mildewed or what have you. I mean if you find one cheap it's worth a shot but finding one is only part of the way there.
I once "inherited" (long story involving a Mormon income tax scam) a pair of EV Eliminator I copies (the original folded horn design from like 1969) which lucky for me had 15" D-130's in them. The guy who ripped me off who owned them had no idea what they had in them.
That was the good part, the bad part was they were totally dry rotted from sitting in a garage, but being D-130's well worth reconing and the kits existed.
As far as new 18" speakers FOR BASS, they have been out of fashion for so long what is actually left on the market? Hell even for PA use 18" speakers have been largely out of favor now for what, 20+ years?
18" speakers still live on for the car stereo crowd (who like to fill their trunk with an entire subwoofer) and some people at home making 18" subs from kits but of course those speakers would be wildly inappropriate.
Eminence doesn't even offer an 18" for bass guitar anymore, they have a plethora of PA 18" speakers but I don't know how well those would work in this Sunn cabinet.
Unlike the old Acoustic stuff there is not a large "preservation society" for Sunn gear which might have specs and other details on these cabinets.
IIRC when Entwhistle used these as part of a mega system for stadiums he might have replaced the 18" drivers with Gauss drivers but I might be remembering that wrong.
Anyway Gauss you wind up in the same situation; recone kits are hard to source and any used ones you come across will have seen better days.
I am thinking the best solution here is to just find an 18" driver that is "close" to the original and just go with that; I don't get the impression the OP is going to use this for a critical gig situation, more to have it and fix it up.
Analogeezer
p.s. I actually own a piece of vintage Sunn gear! It's a single 15" and horn PA cabinet, kinda small from the Sunn era in the 1980's where they painted the fronts of the cabinets white with a see through black grill cover.
It was given to me by the keyboard player in my band, so I would up putting in a different horn, with a different compression driver, a new crossover, and an Emminence Kappa 15"; ported to the specs recommended by Emminence (the stock cabinet was sealed).
It actually sounds pretty good, I set it up as a monitor for the keyboard player in our rehearsal space and he commented on how much better it sounded.....which was not a big deal as I imagine the stock, cheap PA cab sounded horrible.
I've also used it as a monitor for a drummer, but not putting actual drums through it (well electronic drums), has worked well for that.
But I took the grill off, painted the white front and now all the Sunn that is left is the shell, tolex and the strap handle on top.
