Running a older Briggs 20hp on an older style no neutral MB surface drive and I'd like to see if I can get a little more power outta this little motor without killing it, any suggestions would be much appreciated.

Carb and exhaust. Or mod the stock exhaust. Take out the baffles. All cheap stuff. Cuz in reality the mph gains won't be substantial enough for all the money you put into it. Best thing would be get a larger hp engine.