“You need some 13:1 pistons for that Briggs and Stratton, LOL.”
Many moons ago, I milled the head, a bit at a time, on my go-kart till the valves started hitting the head. I never could detect a lot of increase in performance.
I think the problem is a flathead engine can’t really respond to the improvement. Did you port the block to maintain the cross section between the valves and cylinder for gas flow? Not sure how much wall thickness there is in there also before you break thru.