I have modified one engine. It was K&B Series 75 35 used for CL Midwest Sport Race. I opened the crank timing to the same numbers as the K&B 5.8, Raised the top of the exhaust ports, Cut away the part of the piston skirt which covered the bottom of the transfer passage at BDC. Using same prop, fuel, etc, my airplane went 10 MPH faster. I am now retired from the engine modification business--quit while I am ahead.