Any old steel will be fine. A scrap steel piece from your buddy's structural welding truck will work. He'll more than likely know the weld settings.
There's many ways to attach the extra piece, I would maybe drill some big plug welding holes into the added piece instead of the original tab, if I could remove the triple clamp and stand the frame up vertical. Or cut a rectangular piece, a little smaller length/width, weld to the bottom of the original tab on 3 sides, and grind to suit.
Disconnect the battery and unplug the electronic ignition module.
Put the work clamp near to the weld, sand off some paint to make a good contact.
Maybe repack the bearing?