TIM W. - Lucas is correct, as long as you stick with a CC spec block the 10, 12, 14, & 16 HP K-series Kohlers all drop into a WF CC without any changes. Even the K161(7hp) & K181(8hp) drop in and use the same clutch hook-ups, mounting holes, linkages, cables, wires, etc. You'll have to attach the clutch components, 3-pin driver, clutch friction disc, anti-rattle springs, etc to the flywheel end, and the PTO clutch to the shaft end. A Quiet line spec engine won't work as easily because of the block designed to use the Bendiz style starter & elec. PTO clutch.
There was a "K361", 18 HP K-series but it was an over-head valve conversion on the K341 16 HP, you might be able to add the OHV head & ect to an IH spec K341. It'd be an expensive engine! And I can't say as I've EVER seen a piece of equipment with the K361 engine in it. It's pretty rare.
The cyl head on the K341 (16hp) has ten head bolts/studs while the K241/301/321 only have nine and the cooling shroud for the head is different also, and the cyl bore is 3-3/4" while the K321 is only 3-1/2", but the crankshaft, rod, block, cam, governor, etc EXCEPT for the added head bolt/stud is all the same.
And Lucas is also correct, I built up a K321 (14 HP) with the smaller K301 (12 hp) flywheel, back plate & blower housing and dropped it into my NF #72 about 5-6 yrs ago. I didn't want to cut & modify my frame for blower housing clearance like IH did on the #147's. It's more stealthy that way too. Looks exactly like a K301 12 hp. Even the blower housing serial number tags say K301, because I swapped them with the K301 I had in my 129 at the time.