I spent my life welding.
If that was my crankshaft I would get it x rayed for cracks. See if it is worth repairing.
But welding always has consequences. Like weakness in the HAZ (heat affected zone) and possible warpage, and on such a small crank it is not worth it.
I would replace the crank if it needs it, and change that 12mm bolt to a 10mm bolt.
I mean think about this .
What is better a original crank that has problems and could possibly fail even if you repair it, or a new crank that someone with a magnifying glass could say"it's not original.
I would want the NEW crank that looks like the original.
After all it is inside the engine and no one can see it. It is not like an aftermarket part on the outside of the bike that is obvious.
What good is a complete restoration that you would be afraid to ride?