I like the way its done in speedway. There is two rules, engine size and it has to be a speedwaybike. That's it as far as rules. Then run 2 or 3 different divisions 1,2,+3. One is a pro class, the other semi pro and you have a beginner class. Usually after guys get some experience they will want to move up and wont need to be moved up a division. Just incase after a set number of main event wins in the lower classes you get moved up.
The more I read all this midget confusion the more I know were involved in the best racing there is for equality, (not to mention the outright balls to the walls racing) 25% race vehicle / 75% rider output. At the end of the night you can be assured the best rider that night won the main, not the best setup guy.