I found other differences as well, mostly to comply with CARB rules (California Air Resource Board)
like compression reduction slits on top of ports and like 7000 rpm limiter in ignition. The last one was a tough nut to crack. The engine just would not go past 7000 rpm, until I fitted an electronic ignition.
I then checked the patent pending number at USPTO.gov and found out that there were integrated electronics in the ignition coil that regulated the timing and max rpm.