You could modify the X-Wedge with S&S upgrades. It would be more powerful, louder and would need more frequent rebuilding due to the stresses added. Mostly racers do this and longevity is not the reason to do this. You then arrive at the bleeding edge that marks the limits of an air cooled v-twin.
Dan's suggestion of a Miata engine would be sweet. Then you would have a powerplant with torque curves that would mesh smoothly with the Miata's transmission. The car would be quiet and vibration free. The entire front end would have to be reworked for the inclusion of a radiator, extra weight and the removal of the Centa torque convertor. It would look not near as cool as the current model. The steering would have to be reworked for real car tires. That's only the beginning...
