I'm woundering if alittle tweeking would make the 4 stroke faster.The briggs motor I have has bigger valves,duel springs,higher lift cam,milled head,hotter coil,and a stepped exhaust to build power.Not to mention a custom intake,Tiltson carb,and fuel pump.It was originaly rated at 5 hp,but with some tunning.With the right fuel they can produce around 16 hp modestly.I've seen them dynoed up to 24 hp on methonal.
They do the same things to the overhead valve motors,and to both the Briggs,and Honda's. Most of the parts can be bought through Ebay pretty cheap too.I'm woundering if your 4 stroke motorplate can be adapted to alittle larger motor hehe

NOT THAT BIG !!!!