There are several welding process and directions to take in doing the repair especially if you plan on using the repaired part long enough to make a new part. If I were welding it I would go with a nickel rod (as a permanent repair) as a first choice, TIG weld it up with silicon bronze as a second choice and braze it up as a last choice. If you have the room/clearance where the part is located, just fasten (drill and tap) a piece of bar stock on the side to limp through to the next step. In the mean time; As you get it installed a replacement will show up on EBay.
The problem in most shops is finding someone (employee) who can move between projects without two hours of instruction. I get the question; Do you do jobs this small? I work by the hour, so they're all small jobs. You get a quote, I get a deposit and there's no surprises later.
If you nickel rod tig weld that part up
You will never get the taper pin reamed back right.
Sure it will holed good but you are going to wind up with a very hard glaze seam Right where your cracked off piece is and it is right through
That taper hole .
You are going to have to brake that cast iron with
A Yellow brass none fluxed brazing rod .
And buy your brazing flux in a dry powder can
It is Peterson brazing flux . It is a blue dry powder
Flux in a one lb. Can the brazing rod is solid brass slick no flux on the rod .
You will heat the tip of that rod and dip it into the can of flux it flux will stick to the end of the rod
Then you will keep doing that as long as you are brazing the first pass .
Clean all of the hardened flux before starting the next pass you do this until you finish .
Then you can machine , drill and tap , and bore , and ream your taper hole without any trouble.
The Yellow brass brazing in this way is by far the very best way to repair the Broken clapper box .
Bar None .
You are going to have to buy the reamer also