So after building up all the missing metal the next step is turning the shaft back into dimension.
To do this properly a lathe and/or a special grinder is used. I have neither but the points from that compass, a dremel and a bit of ingenuity.
Thus, this is how we create a precision ground shaft in the third world:
I did most of the hard work free hand at the bench grinder and then used this contraption and a small stone on the dremel to get it as close as possible to how it is supposed to be. The keyway only needed a small touch with a round burr after filling the walls.
The gear fits nice with just a bit of runout that I hope the rest of the tolerances will mask out.
And after putting all back together it’s running again. I only needed to add a shim between the gear and the bearing to take out a bit of axial play.
When it broke I was almost about to dump it, I had little experience welding and back then it was really cheap. But in spite of how it looks on the outside it has a very sturdy construction (for its price) and a quality needle bearing instead of a plain bushing like the others.
It was a good call, as now I have another working tool.