I'm actually surprised that it runs at all naturally aspirated, considering how small the valves are. (remember, when you cut a diameter in half, you cut the flow rate by 3/4).
I would expect it to go the other way, as another consequence of the square-cube law. Making everything half-size gives you 1/2
2 the flow rate, as you say, but the cylinder volume would be 1/2
3, so the smaller you get, the
easier I would expect aspiration to be.
Or I'm smoking crack again.
-BP