Hi teddy,
When you lowered the cylinder, you also lowered the transfer ports as well as the exhust. I beleive that the small transfers are the weak link in the 1bbl engine. Also the piston skirt blocks the transfer passages at the bottom of the stroke. If you havn't raised the transfer ports, the their timing is very retarded now, as it was only 51 degrees BBDC stock. You could have a rpm limiter circuit in your coils, but it doesn't sound likely, since you tried several. Have you ever got over 6500 rpm with any Toro? Here is a G23/26 piston pic.

Dave