The most honest way to "level the playing field" is to remove the car rules entirely.
Unlimited engines, number of cylinders, displacement, country of origin, material, method of induction and electronic controls and so on. This insures that anyone may use what they feel is appropriate for a given track.
Scrap the weight minimum as well, why should a person be penalized by weight? I weigh 145 Lb's, should I be required to wear a a 100 Lb weight when competeing against a 240 Lb person as if I were Harrison Bergeron?
This had the added benefit that a track requires no paid inspectors or measuring equipment.