There are alternative rods for welding cast iron which mean you don't need to do the old "preheat\post heat" thing. They are usually high nickel, and will look rather different. The weld is usually nice and smooth. Some rods become very hard and can only be ground, not filed. Talk to the welder or welding supplier.
The technique is different too. Once the edge prep is done, the new section is shaped, positioned and fixtured. Leave any excess sticking out, as is for the time being. In case of movement, you have some spare to shape - also helps some to reduce the risk of cracking.
Some people liked to have a copper heat soak underneath, I never usually bothered, but it can assist controlling penetration.
Only a short bead is laid down each time, maybe a 1\2" - 1cm. The part must then be allowed to cool naturally to no more than hand hot, before the next bead - I would alternate ends. No forced cooling, we used an old fire blanket to prevent draughts and slow down the cooling.
Fortunately, it is the top fin, so access is easy at least. In the old days there were usually plenty of scrap cylinders around to bash a section of fin from, probably not so much these days.
I have done down and dirty, with low carbon steel and a mig welder. The mig allows a very speedy bead to be laid down. On a long run like the example I would maybe try an interrupted arc. A mig bead is usually nice and ductile.
But leave the patch piece as large as possible to try to balance out the heat soak.
Works OK, but low carbon steel tends to look different, even after blasting.
Leave the thing alone after welding until completely cold.
And if you hear a ping or a snap, you got it wrong.
