A good Homie 30cc with a modest amount of work will turn a 18x8 prop in the 7700-7900 rpm range. The 30 likes to rev up so prop it for 7500+ on the ground
1. plug the hole its costing you 100 rpm or so.
2. Larger carb about 11-12mm will add 200-300 rpm. Note to big a carb and to big a prop is a BAD IDEA.
3. Electronic ignition adds 100-200 rpm. You can set the timing for max power and you remove the drag form the magnets and the drag from fan on the flywheelng
These are the easy big improvents. The rest offers smaller improvements for the effort