I am using tig as that is all I currently have available but mig would work as well with small wire. The key with the tig is to focus the arc on the cast yoke, a light dab of filler, wash the bead onto the cap then immediately get out of it and force cool.
Another option could be to plate in like below and perhaps there are already washers of the correct 15mm size out there. Welding caps is apparently very common for the 4x4 guys, and they do it on high speed applications like driveshafts/axles. Worst case I already know it's easily serviceable as I had to get the poor quality joint out. I'm really hoping this bearing will last another 50 years like the original 😂 but we will see.