Why cant you port the cylinder head, raise the compression ratio, port the exhaust system, and intake system, so air can travel though pipes smoother/faster all way to engine.
You also can do lots of thing on carb for the starter. examples:- Knife edged, thinner spindle, ported throttle ports, etc. the gain wont be much, but it all add up from modifcation there and that.
its free to do with dremel and bit of your time. but you have to know what you doing.