well increasing max displacement might be feasible for a mod but making a functional radial engine type of thing would require extensive knowledge of the mechanics-something that would have to be undertaken by the devs, and i think there are other higher priorities for them. but i think theres an engine visual designer mod you could use to make something look right.

Depends on what time of aircraft engine. You can make inlines, Vs, and boxer engines. But I don't think we will see much more outside of those.
https://en.wikipedia.org/wiki/Chrysler_IV-2220#General_characteristics
I don't know much about airplane engines in particular, but one I do remember reading of was this turbo and supercharged hemi v16 that chrysler made. I don't know how many planes (if any) used this though.

There are v16s in game (dlc) and you can get them up to 21liters or so iirc. Turbo it with high octane fuel and you can probably get similar power results to that chrysler one. I am not sure if you can get those results in 1946, maybe later, but it might be interesting to see.