Have a single spec engine available for purchase or rebuild from pre-approved suppliers, who then seal the motor. LOTS of 06-08 600cc motors available- F600 would not even scratch the surface. It would be fair, inexpensive, and easy to police.
That's the way FE does it, right? I know it works for Rotax TAG karts.
This would be a logical step-up from kart racing with a slightly larger, more powerful, and safer car. The current pull-start CVT F500's just are not sexy.