Robert Beale: Your post piqued my interest. I appreciate your posting the pictures. I wasn't even aware of the CH20's existence until I saw your post. I re-powered a 782 with a Magnum 18. For everyone's edification I've posted the specifications of the original KT17, your CH20, and my M18.
I find it interesting that the lowest RATED engine (KT17) had the largest bore the longest stroke (although the M18 equaled it).
Also note that there was no difference between the KTxx Series I and Series II in their basic specification data.
As I'm sure you're aware, there are specific height and distance issues when installing an engine in the Cub Cadet. The tail shaft of the engine needs to be slightly elevated in relation to the input shaft of the transmission. Since you have the KT17 as a guide, just make sure it is at the same height and in the same relationship to the mounting plate fore-and-aft.
By the way, I notice you already have the plate installed on the engine --that is the best way to mount the engine. Been there, done it the other way, --not pretty.
Finally, a posting note. If the Forum Police are on the prowl, and if they're not in a good mood, and if your TRACTOR is not a genuine IH built Cub Cadet (Model 782's manufactured by Cub Cadet Corporation don't count), you may be asked to post in the CCC/MTD section. I speak from experience.