I doubt you're going to get the degree of variation you seem to want; one or the other is going to be optimal. Even if you manage to balance the displacement limits to give equivalent power and/or power:weight ratio to blown versus NA cars, the NA is going to have an advantage in reliability due to fewer moving parts; in all other circumstances one or the other will have a performance advantage.An outright ban on blowers (or turbos) isn't something I'd prefer: the more variation, the merrier.
I'd actually prefer a ruleset that would incentivise or at least not overly penalize blown engines, as that's a technology I'd want to see advanced for the spillover benefits to street cars.
That's...I mean, it sounds cool, but the mind boggles a little bit. Radials are an absolutely godawful layout for a groundcar engine- greater frontal area and higher center of gravity than virtually any inline layout of equivalent displacement. Why would anyone actually do that? Was a radial just the best engine available to them so they decided to build a car around it?An Italian prewar experimental was powered with a two cycle 16 cylinder air cooled radial. Neat front-wheel drive car with NACA engine cowling. Ran, but never developed.