Hey, I did exactly the same thing. The nut is a reverse thread nut, and the shaft is just weak enough that it will twist right off. At least by your picture, you have a stub to work with. With mine, it was right down to the point that the tip was pretty much sheared off the end of the armature shaft. In other words almost flush at the tip. A friend with some decent tools helped me out. First, we filed the remaining stub off so that the end of the armature shaft was perfectly flat. Tapped it with a center punch and drilled it down with a drill press and tapped it for a #8-32 screw. Although the original was a reverse thread, all I had was a standard direction screw and tap, so that's what we used.
Re-assembled it using some nail polish on the threads and a small lock washer to help hold it in place since this was not a reverse thread. The important thing is to have it exactly flat. Adjustment was a little dicey since the top to bottom position of the ratchet has now changed in relation to the tip of the diaphragm. Also, on mine there is a little wobble to it which makes for a unique oogah sound, but for me its better for now than nothing at all.
Since this was only a week or so ago that it was done, I don't know how long it will hold with the lack of a reverse thread.
At least for now it works and that beats the cost of a replacement horn.
I called Bert's and he said that he is trying to gather up enough of these to send off to someone who does the repairs with an actual reverse thread post, but based on the numbers he gave me in terms of how many he has and how many he needs to be able to have it done for a reasonable price, it may be many months before he can get enough to make it worth while.
So, at least now my oogah is working, but for how long is anyone's guess. I am not saying this is the way anyone else should fix the problem, but it is the way I did it.