You need to also think about valve float and bounce. I know the valves on these little machines are small, so do not have much mass, but also the springs are pretty light. The critical point is at overlap of the intake and exhaust valves just prior to the induction stroke. This is when piston to valve contact can occur. Any issues with the valve timing (valve not being able to faithfully follow the cam profile) at high rpms could result in contact.