Ask the Cue Maker Forum is a good bet. Not that the members in the Main can't answer your question. You have the makers who can answer your technical questions and players that have been there, done that.
If the shaft is that old, I might hesitate to monkey with it just out of principle.
if you are planning on giving it to a repair man, he isn't going to take it all the way down for you in one shot anyway, which is probably a good thing anyway if you are trying to find the right diameter that you are looking for.
For example, you can buy a Players LD HXT shaft on EBay or any of the other suppliers for a real decent price. I got a great deal on a Players HXT, 2 of the 12.75 and one 11.75.
You can get them in a 12.75 mm and 11.75 mm just like the Predator diameters.
Myself, I can't play with a 13 mm any more. It feels like I have a telephone pole in my bridge hand.
I have and play with anything from a 11.75 up to a 12.40.
Then, along with finding the proper diameter that you like, you will most likely be interested in finding the right taper for yourself.
If you were to try a after market shaft, all you may be interested in doing after is having a repair man play with the taper a bit for you once you find the diameter that you prefer.
Once you find that magic diameter and taper for yourself, it will be much easier to have a maker or repair man duplicate a spare for yourself in the future.
Keeping in mind that most likely, unless you are lucky, you won't get 2 identical shafts that are going to feel or play exactly the same for you.
Makers can try and match them up and they may get close but then you are buying very good quality and that will cost you more in order to have that done.
Depends on what your budget is for a couple of shafts.
Myself, I'm not that great a player that I can discern between two shafts anyway. I can to a certain point but its so minute that the difference between 2 shafts isn't going to affect my game much anyway.