Check for tight valve guides, or varnish on the valve stems.
One year I parked my lawn tractor in the fall, and it ran fine; after setting all winter I went to start it up and one cylinder was dead. The valve stem had varnished up from sitting. The valve moved a little bit, but it was being held off of the seat by varnish.
One would think that removing the spring and spraying the stem with carb cleaner would dissolve the varnish. Well after about an hour of spraying and fighting with the valve I finally got the varnish off enough so that the valve would seat. In all my 5 decades of working on engines I have never seen a valve stuck that badly. I've never seen it happen since that time.
The relatively weak valve springs would never overcome how badly that varnish had that valve stuck. That was about 15 years ago, the engine ran fine until the deck finally rusted out. I still have the tractor, likely the engine is still in good shape.
Just one freak thing I ran into over the years, maybe it's not your problem, but at least something to consider looking at.
BTW, my neighbor owns a lawn care service and runs Toro commercial equipment. He told me that everyone he knew that purchased mowers with that proprietary engine has had tons of trouble with them. The new Kohlers are supposed to be problematic also. All his mowers have Kawasaki engines on them with tons of hours on them. little to no downtime. Last zero turn I bought I also went with a Kawasaki.
Good luck solving the problem......
