An old thread... but such a great question and so I'll chime in. The Honda GCV engines might be best OHV mower engines per your requirements, as I have discovered. It's not only OHV but also OHC - a design that eliminates much of the valve train slop making for a cleaner, more efficient engine. I have a GCV190 mounted on my Snapper 21500PC with quadra cut blades, love it. There was some question regarding the durability of the cam belt, but my 10 year old GCV outlived it's original HRX deck and gearbox before powering my Snapper, still runs great.