The intent of the rule is that we in IMAC fly a scale model of an aircraft that has been built and is capable and legal in FAA terms to fly an IAC sequence within the aerobatic box. This opens the door to many homebuilts, racers, manufactured GA aircraft, and some warbirds.
There are indeed many variations of Pitts, Eagles, Lasers, Skybolts and so on. So generally you will not have any problem if it's within the normal mods that the full scale guys do. On the other hand if your model is "very different" than what one would normally expect from that type, like maybe a Pitts fuse with a Edge wing and a Skybolt tail group, you better be able to show documentation that such a full scale did exists.
When choosing a type to model, you want to make sure that the type is certified for aerobatics, including spins, and that it would be able to perform a sequence within the 3300 ft box.

Tom Wheeler
IMAC President