I have around 65+ years experience with Metalworking, and I'd suggest getting it welded up and getting the keyway re-machined. No big deal with a guy who knows metal properties, and is a good weldor.
Or, machine a brand new arbor from bar stock.
Anything less, is a temporary fix. IOW, you will have to have it redone one of these years.
This is exactly what I was thinking I would do, before I got to this post. It is not timed so where that keyway is cut is not important rotation-wise. It could be welded up as filler (The old slot) and the keyway cut into the opposite side. They could bring the arbor flange into true while they are at it...
I have to say that with rebuilding old iron woodworking machinery and building new jigs and "things" for my other woodworking tools, I do my own welding, but I do have a relationship with a machine shop... The main guy there seems to have a curious interest in the things I come up with. He always meets me at the office with a pad and pencils and a smile... Asking what we are doing now.
Example- my latest is having a router chuck adapter for my RAS machined out of a Porter Cable 890 arbor shaft... He wants the power unit there to check the fit... 20,000 rpm tool speed. He want's to make sure eveything lines up. That's a good thing.
***If for you, If you can get an arbor new. It's going to be cheaper than to have it welded and machined. Machine shops are generally expensive. I understand that I tend to get the entertainment value, veteran/legacy old-timer discount.
But even new, I check my new arbors with a dial indicator. You would be surprised how much some new arbors are off (Quality Control). Easy to dress it to bring them back in. .005" off on the arbor flange can mean a whole lot more off further out on a 10" or 12" blade... And you would be surprised in how much a better cut, less noise and vibration there is when you do than.
Remember, since you took the trunions loose... When you get it back together- re-align and true your saw to the left miter slot, both at 0 and 45.