I've replaced more than a few 'O' rings. I like to take the pump cover and run it over a flat mill file to make sure it's flat and then a new 80 cent 'O' ring and it's off to the races. One thing about the cover is, there is only one 'right way'. The wrong way and the mower don't move.
I've got over 1250 hours on my 50" tank with very little issues, other than blades, a spindle once in a while and deck drive belt. The kawasaki is a pig on gas however. My wife's Kohler is much better but the Kohler's valves tend to need adjusted every year. Been letting the wife do most of the mowing as hers is, like I said, much more frugal on fuel.
I've heard that 1000 hours is about all one can expect on the Kawasaki but I've really had no engine issues at all.