As an engine machine shop owner of several decades, I would suffice a guess that the engine shroud was holding debris around the cooling fins of the head and thus overheating the entire combustion chamber as well as the head.
Once an "air cooled engine" or as I much prefer to call them: "Fuel Cooled Engine" has began to run overly high atmosphere the engine then will require more fuel in order to further cool the atmosphere. If additional fuel is not available, the atmosphere reaches critical mass and many things can and certainly will happen at some point. One such thing is galling the valve stem, reducing and clearance. The valve stem thence collects the valve guide and brings it on the upward or downward travel until something stops them both from moving such as breaking a rocker arm stopping any movement. Perhaps the push rods bend and the movement is much decreased until the push rod slides out of the rocker arm cup. Or the valve is so slow in movement due to the high temperatures the valve stays out the downward travel too long and thereby makes contact with the piston. Just a few of the scenarios you may experience with high temperatures due to lack of cooling either on the outer surfaces such as the cylinder cooling fins or the interior of the cylinder due to insufficient fuel.
Next thing to do is read the plugs. Are they reading the same on both sides, or, the one with the migrated valve guide, does it appear to have been extremely hot?
Boyeeeeeeeeee, we could go on for a while. I'll leave this to Bert to pick up as he will be a bit more technical than I when it comes to fuel cooled engines.
My shop's duty was always to do the autopsies on engines when dropped off for restoration.
Good luck and I don't feel your Kawasaki is the problem, it is something you'll find once all the evidence is in.
Max