Honestly, I don't know, but I think if you go bigger, you'll get less power.
There are other things that affect the RPM vs power curve, but if you look at the curve for the stock Puch setup, it's pretty linear from idle to a little over 2000RPM, then it starts to curve. As the RPM increases from 6000 to 12000, there's not much change in power.
There are a few things that contribute to this, like the source resistance of the coil, the load resistance of the lamps, the inductance of the coil, and maybe partially because of saturation of the core.
I don't know if making the core's cross section smaller is going to help or hurt, but I think going bigger isn't going to help, since you can't fit as much wire in there,