Dan,
I think some Epoxy / filler will do a good job for your situation. I did that same repair on my old H16 a few years ago, using West System Epoxy. Epoxy isn't the easiest thing to sand, so don't expect to put it on thick and then carve it down to shape. Try to get the first thickened coat applied in the general shape of the original keel. You can go back and fill in any defects later. Epoxy doesn't hold up to sunlight, so you'll have to paint it. I was able to find spray paint to match my boat and the repair wasn't very noticable at all.
Jack