Well, I looked a little harder and I can still buy the longer one, it seems, so I cut the longer one in half using a hacksaw and the jaws of my lathe as a guide. The cut was pretty straight and will yield 2 shorter shafts.
I milled the rough edge to just 0.15mm shy of the 18mm I think the other one was (can't get a perfect measurement from a broken cast part). Why? Because it was a super tight fit before. It has room to move.
Then I filed the sharp edges and put a bevel on the splines, freehand.
After all that I went to fit it in the gears and the replacement part seems defective. One of the splines won't engage when the other three do. I tried all of the possible rotations and it just won't engage.
There's nothing too critical about the fitment so I'm going to hand dress the offending spline. It should be OK.
I'll be buying a pair of the long pieces soon in case they get discontinued also.
Also on the list will be a 4 jaw chuck. I had to do the bevels by hand because it's impossible to put this part in a 3 jaw chuck.
The bevel on the left is the factory one. The hand-ground bevel is on the right.
I'm still going to look into better options going forward because this is a mistake I've made 3 times, now. Two broken gears and a broken taper pin. You'd think I'd learn but I'm sure I'll make this mistake again.