Just a thought Cyril, and maybe a lot of work?
I know you are pretty handy though. You could make a 2 piece extension that clamps onto your valve base on one end, and has a thread (same as the adjustment thread on the cable). You can turn it up in the lathe but leave 2 "ears" or a larger diameter on the piece. Make it out of something easy to machine like Aluminum. Plan on slicing it in half (after all the work is done) so you can clamp it back on. You would drill and tap the ears. One half of the ear would have the tapped portion and the other half of the ear a clearance for a cap screw. When you clamp the piece onto the base, it would be tight, and the threaded part should be the right thread. There would be enough thread to screw in the adjusting thread on the cable, but it would be a 2-piece thread. You might think about sealing it once attached.
Like a I said, a bit of work to make the extension, and maybe not worth it? You might look at what a new correct length cable would cost. However, if you have the time and don't mind the challenge, you could do that and it should work.