I agree with Rob, maybe variable motor options depending on the track/style of racing are what we have to do. Events will just have to pick what speed motor they want to use ahead of time and let everybody know. Make sure a ton of those motors are available for sale at the race, or even for lease or something. Clubs can pick motors based on what's right for them. It seems pretty clear one spec motor isn't right in every situation. Three classes is plenty, just make sure people know the motor for that class will depend on the track/event.
Stock (21.5/17.5)
Spec Mod (13.5/10.5)
Mod (open)