Eddie B.-
I'm in a similar situation to you, I'm putting a KT-17 into a 582. The KT-17 is externally nearly identical to the mag 20 you are working with. Part 12 in your picture won't bolt up to the Kohler, you need one from a 1000 or 1200. Or you can do what I did. If the engine is solid mounted you can use the 70-128 style clutch driver and put a solid coupler in the rear and toss the rag joint because the engine's crankshaft is in the same axis as the input shaft on the transmission. I am making a custom length driveshaft for this, and if you need any more information, let me know, I did a bunch of research on this. Also check out the thread I started in the MTD section about CCC 582's. I have some info in there. Let me know if I need to clarify anything or if you have any more questions.
Almost forgot, to bolt the clutch driver onto that engine you need that aluminum drive cup that bolts to the flywheel end of the crank too...