Harry B - the single cylinder Magnums are definitely cast iron. My back can testify to this.
My understanding of the Magnum is it is essentially a K series engine with some upgrades. Solid state ignition, Mahle pistons, bendix vs S/G starter. I believe they also have satellite valves.
Back in '97-'98 time frame I bought a Wheelhorse tractor new. It was the last model they made with a cast iron engine in it and of course their cast iron tranny. I consider it the last of the old iron tractors. It has a Magnum M14. Around that same time, I bought a Troybilt tiller new with the Magnum M8. For a hack mechanic like myself, I consider the Magnum engines to be superior to the K-series.
What spurred me to make these purchases, besides just needing them was the EPA mandates that I feel helped lead to the demise of the Magnum engines. I was glad to get these machines when I could and use to see direct replacements littered on the shelves of Northern Tool and kick myself for not picking up a spare engine or two when they were available.
Here are a few links on the Magnums:
http://deadsledpulling.weebly.com/engine-rebuilding-1.html
http://www.hancox.co.uk/kohlermagnumsingle.html
It would be great to know for sure if one could drop-in a CCC magnum into an IHCC.
IHCC content: Warming-up in my neck of the woods, looking at using the 126 while I've got it at the house to pull my plug aerator. Don't know if anyone ever uses these, but rather than rely on the hole in the draw-bar tab on the back of the tractor, I use a 1-7/8 ball hitch with a 1/2 inch shank. I think I got these from Northern Tool years ago. I also fit my aerator with a 1-7/8 hitch so I can hook it to the ball.
Around here, there are lots of hills, dips and ditches and the normal clevis on a pull behind implement can bind on the draw bar tab vs pulling on flat ground. I have done the same for pulling trailers too. Works GREAT!!!!