They say Kawasaki is the best engine in the commercial mowing industry but I won't tell you that see too many of them for me drops valve. You never see Kohler or Briggs drop valves. Kawasaki says their goal is to have the best engine in the industry. Well if they want that they need to fix the head problem and then they might have the best in the industry.
I have to disagree that Briggs and Kohler don't a valve. Valve physically in place but either a valve seat or valve guide usually unless it just someone never adjusted the valves.
My personal 33 series Briggs lost the exhaust valve seat. I have valve guides move on both Briggs and Kohler OHV engines. And even had a customer that nearly lost a Briggs engine because a valve seat failure. I ended up having to replace both the head and piston. Usually when the Briggs valve guides moves they do drop a valve from have a bent push rods. Also sometimes a Briggs valve gets dropped due broken push rod (one that worn to thin).
And yes I have replaced two Kawasaki heads this year because of overheating due to block cooling fins. This is not a Kawasaki but is maintenance problem. The first one was cause oil and dirt compaction causing by a flywheel oil leak that the customer ignored. The second one was on a Wright stand on mower which was actually a Wright design problem. They have put the hydro cooling radiator directly over the engine. They were depending the engine to suck the heated air through the engine. Well the radiator became half clogged leading to over heating hydro system which in turn over heated the engine. Actually melted the engine shroud and the flywheel fan along destroying #2 head. Neither of these were Kawasaki design problem but induced failures.