thanks for your input motobee. i was considering something along those lines. the 15mm sha with matching malossi reed valve intake. i'm guessing the malossi variator would give greater gear range (with perhaps higher top speed), but i'm not sure.
i agree it does run well as is, but i sense that the gurtner isn't quite giving enough mixture to the cylinder.
i don't think i want to port the cylinder because that would be cutting or grinding and that would be a permanent modification that i'm not comfortable with.