Flathead,
I have a Lawnboy engine too. I believe you are looking at somewhere around 140-145CCs (cant remember exactly). Ive cut one down and chopped off the shaft for the mower blade. Havent done any more than that, I was worried it would vibrate a plane apart but would still love to try it. If you ever run it start a thread, Ive seen others talk about it in the past but have never seen anyone post any numbers.........mark