The Fox 35 Rules are stable and are not changing, late last year the rules were clarified so that you couldn't use a rear intake / needle valve setup. If the engine looks like a Fox 35 Stunt we don't get all twisted about what liner, head or needle valve is being used. If the person modified the timing that is also hard to inspect at the flying site. The only inspection we do here in St Louis is a 5 second look to make sure it's not using pressure, it has a front intake with needle valve and that it's a Fox 35 Stunt engine
The only thing to separate out the slower and fast airplanes is to have two classes, Beginner and Expert. Once you fly faster than let's say 80 mph you are flying in the Expert Category. I would agree that the mods to the Fox 35 Stunt are well understood, there is still plenty of life left in the Fox 35 Stunt Speed event. I would propose that you have two average speed awards - one for Perky and another for Fox 35 Speed. the average is really what you are trying for!
Having the Average trophy was part of the original Perky event so I don't understand why it's not offered at the Brodak contest. That way people with slower models can shot for the Average award. I certainly do not try for the top speed.
Fred