It's a tough situation. To begin, you'll have to measure the exact internal volume of the cab, then (if the box is ported) measure the port sizes. For round ports: diameter, tube length, and number of tubes. For rectangular ports: height, width, and shelf depth. Then comes the "fun" part: you'll have to use a box simulation application to model various Celestion drivers to see how they perform in that enclosure.
There's a decent discussion from 10 years ago of WinISD vs Bassbox Pro at:
https://www.talkbass.com/threads/bassbox-pro-6-vs-winisd-which-do-you-trust-also-12-driver-recommendation.1130103/
I've been using WinISD 0.7.0.900 for so many years that I know its quirks and how to work around them. But if I were to start over, I'd go with Bassbox Pro.
There's also Speakerbox Lite on the web at:
https://speakerboxlite.com/
...but I can't vouch for its accuracy or lack thereof.
You'll have to decide for yourself whether any of this is worth the effort (headaches). Thinking long term, it might make more sense to retire that combo.
