You could probably get away with either brazing or nickle cast iron arc rod welding for that repair. Some torch preheating required (spit test).
Not nearly as bad as trying to deal with a broken spring pad in the bottom of the pan.

I'd turn it over on its flange and c-clamp to a flat plate to get started. Yes the pieces will need to be v'd.

I'd machine up a plate of cold rolled steel that had a step in it to fill the void that corner leaves, build it so it uses the three corner holes to hold that up in place and seal it. I wouldn't weld or braze on the corner of that pan. I'm superstitious though, and my fear is that when you heat that pan up it will weld fine but cast iron could crack once it shrinks again. You do what you want, it is yours.

Google lock-n-stitch too, that is a good method and I have used it before too, although a replacement pan might be cheaper than the parts from them, but it does work if done correctly. I repaired a D4 pan that had a busted spring seat, the piece was not pushed loose from the pan and I used 1/4-20 cap screws in the same manner as lock-n-stitch. I completely drilled out the cracked areas and they are held in place now with threaded pins. I then sealed it with fuel tank liner painted on the inside and it has yet to leak some 5-6 years now. I got pictures somewhere just can't locate them.

Braze it as it is a no load spot. Go to TSC to get Brazing Rod in small quantity. Don't procrastinate on fixing it as the Cast is still clean, no Oxides growing in there. It will repair much quicker and easier if done soon!

How about a combo approach, make a steel stepped block that both fills the empty spot and acts as a doubler on the outside. Braze in place, or use as a clamp with sealer.

I'd use the Electric rod for cast iron as OM says. V out the outside but not so much that the pieces don't fit together securely. Use a straight edge to be sure the machined surfaces remain aligned and clamp it tightly. I'm a big fan of preheating but others say cold welding is Ok. I'd get the big Barbie out and get it as hot as I could. Weld the outside surface first to hold the pieces together, then grind out a V channel along the crack line on the machined surface side and lay a bead filling the V groove. After welding, keep the Barbie going slowly reducing the heat. Some guys maintain burying the part in the earth to let it cool slowly but I find it not necessary. In the end you want a good gasket surface, so you'll need to do a nice job of grinding and filing the machined surface checking with a straight edge. Plenty of gasket sealer in that area going back together is advised.
(You might not need to get that crazy because you almost have enough gasket surface left where you might not need the pieces at all, but you'll be sick if it leaked oil there.)

So how did you drop it? Carrying it or trying to get it out from under the Cat? That thing weighs 105 lbs and is a real bear to get out when you are all alone out in the bush lying on your back under the tractor like I was this past summer, but I got it out, painted and back on during a field ring job.

i would say go with braze as if something really bad happens and things don't line up you can re-heat / re-flow it. one other good thing about brazing is it will go a little slower and you can just push the braze around with the flame of your torch if it not liking to go some place give it some more flux when things are good and clean and flux well it will go up hill by it self. take your time and have fun.