When I was in my last semester of college as an Industrial Arts major, I had to take a small engines class and rebuild a lawnmower engine for a class project. I used a B&S flathead mower engine, and since I never could resist hopping up an engine, I ported and relieved the little bugger while I was rebuilding it. Long story short, in stock form it put out 4 hp on the dyno, and after my "improvements", it put out 5 hp, an increase of 20%(or maybe 25% depending on how you look at it). You might get more results from improving the fuel/air passages in the block rather than redesigning the combustion chamber. Or at least you could possibly get more efficiency from improved breathing in addition to improving the combustion chamber design.
