Now I think I understand where you're coming from. It would be the same as control line for SPA with no modifications. There is some wiggle room but not much. It is frowned upon.

We had a situation in SPA where some planes were being purpose built by a very talented builder to perform pattern flying better than the original design. They were really light, lengthened, the stabs and control surfaces were modified, and the airfoils might have been tweaked. The plane no longer looked like the original design. The governing body allowed them to use their aircraft till <Date> and then they would have to replace them with a period aircraft.

Who decides at the event if it’s ‘illegal’? I guess that could be the Contest Director. I personally wouldn't make the ultimate decision, I would want to put it to some kind of vote but if you show up with a 'Duck' and you say it's a 'Goose' I would have to say that kind of ruins what we're trying to do which is to replicate the period aircraft and flying style. If these guys were in the Novice class or Sportsman class I don't think it would make as much of a difference but if you're flying in an Expert class your skills should be good enough that everyone is pretty close to same but the plane could affect who wins. Each plane has it's characteristics and you have to practice enough to know how it affects the maneuvers and adjust accordingly. If a contestant showed up with a plane that looked like it was modified I would mention it to them and probably myself and the other regional contest directors would debate rather we would disallow it for future competitions. But it would have to be a serious deviation before that would happen. SPA has a governing body but I still think contest directors could step in, but don’t quote me on that, we haven’t had to do that yet.

There is a lot of talk about engines, weights, and airfoils in SPA. Because of the limitations imposed in SPA, they are pushing all the limits they can. To be honest, the reason I am trying to increase performance and decrease the weight is because the maneuvers will be easier. CPA might not have that problem because they don’t have the restrictions.

It's also no secret that the wing drawing you get is probably not how your foam cores are cut. I’ve heard some designers kept the actual airfoil secret. So what are you supposed to do if you don’t have the original profile? Get it close or ask someone for the original profiles I guess. I think this may be part of your question. There are a lot of NACA airfoils or pattern airfoils that might work better than the original. I don't think anyone will ever know but I personally would try to get it as close to the original as I can. So if you’re asking can I change the airfoil to one I know works better. You shouldn’t but I don’t think anyone will know. If you want to tweak a design or use something more modern the classic pattern contests are not what that’s about. To me it’s like putting an engine in a soap box derby car.
Hopefully somewhere in this I’ve answered your question.

Tim