Agreed, pre and post heat are important. Perhaps stress relieving as well.
I think it's helpful that the keyway doesn't dead end in the part, but cracking is still a concern.
Looking into the future... if this repair weld fails under use and the key pops out, what will be the result? Lives at risk? High dollar equipment failure? If it's no big deal, that's one thing. If the loose key winds up stripping the teeth from a million dollar reduction gear, that's another thing all together.
A typical PQR of end choice filler / parameters would be good, but wouldn't necessarily replicate the experience this repair will undergo.
I wouldn't proceed without a PQR though. After that, it's a gut check call comparing the cost savings of repairing the material, vs the risk of what happens in the case of a failure.
Perhaps someone else will pop in that has been in this exact scenario before.
My personal experience with weld repairs on this material, which was cracked fillet welds only, is that it is a crap shoot.
Tim