Can't really tell the scope of your damage without pics, but here are some things to consider:
-If the CI has been exposed for a couple of years then salwater intrusion and corrosion will have traveled and you will have a lot of rust to grind/blast away. It may not be limited to the keel either.
-Epoxy is the usual method to fill and fair. How big a chunk is missing and do you need the ballast it provided. If not, scabbing in a chunk of CI sounds like a difficult solution to a non-problem.
-If you do need to use a piece of CI, welding is difficult at best, if it's even possible... Pins and epoxy would be my first thought.
-Any impact sufficient to damage an Iron keel could very well cause other problems that may remain hidden. If it's a bolt-on, you might want to have a look at you keel bolts.
Fortunately you are not the first guy trying to figure this out. Google turns up tons of 'How to's,' including this from sailnet: