I would strongly advise not having a custom die made, even if it were cheap. Which of course it won't be. I doubt even Campagnolo has one of those weird M10 x 26tpi dies, since their axle threads (back then) were rolled, not cut. The other M10 x 26 tpi thread they made, the screw that attaches the R. der. to the dropout, was almost certainly made on an automatic screw machine using a single-point tool, not a die.

But lets say you bit the bullet and had a custom die made, now what? The problem is, how do you start a threading die on a screw with the first couple threads mangled? There's no way to start it in-phase with the good threads farther down, so the die will probably start cutting out of phase with the good threads, and ruin all the threads, even the previously-good ones. I'm not even talking about the problem of starting the die aligned with the axis of the part; that can be achieved on a lathe, or with some sort of custom-made pilot on the die holder (but you have to make the pilot, it's not a stock part anyone sells). But no amount of skill or technology will overcome this phasing problem; only blind luck will save you, maybe one out of a thousand tries.

The only way I know to do this precisely (not counting the thread file or triangle file, which are only approximate) is on a lathe, with a single-point cutting tool. I could do that, but that's putting $100 worth of labor into saving a $20 part. Even though I have a lathe, I still would fix that axle with a triangle file, or by just forcing the cone over it as I mentioned earlier. Or just cut the mangled end off, and re-center the axle with less stickout on both ends.

Maybe it could be done with a segmented die that can be expanded to go over the bad threads, then closed onto the good threads, and run in reverse to re-cut the mangled thread. If one of those existed in M10 x 26 tpi, which they almost certainly do not. I have no experience with those 3-part segmented dies, only heard of them, but I believe they are large and super-expensive, and run on a dedicated machine. But now I'm being ridiculous, no one is going to spend five or six figures to make a custom tool to repair that $20 part.

The only segmented expanding die I've even heard of was owned by Tesch, who made his forks in big batches with long unthreaded steerers, the cut them to length as needed and added the threads last. As you may know, 1" x 24 tpi threading dies exist, but pretty often they screw up (no pun intended) and absolutely ruin all the threads. They cut beautifully going down, but then something goes wrong while unthreading the die back up. The expanding die gets around that, and saves time, by opening after the threads are cut, never backing up. The shop where I worked sometimes sent a fork to Dave to have that done, if the fork was valuable enough to be worth the round-trip shipping and the time it took. Not a service he offered to anyone, but we had a special relationship because he bought his lugs, BB shells and fork crowns from us. He saved our bacon a few times. Or at least we didn't have to make a new fork and paint it to match, sometimes the only other choice.

I still like threaded headsets and quill stems, for sentimental reasons, but man what a PITA those steerer threads were sometimes!