I'm actually having real-life problems with this as well. I have a Piper Aztec-F which has twin 250hp Lycomings. The lack of air density at higher altitudes makes the air-cooled engines run hot; the fins on the cylinder heads just don't have enough air molecules impacting them to carry away the heat. This is at relatively low alts of FL 180 - FL 200, not at the 30k + alts that an interceptor would be required to fly at!
I wonder if Jugs/Corsairs/Hellcats had any issues with this. Did the Pony and Lightning designers opt for liquid-cooling because of the altitudes they were designed to fly at? I imagine that a radial engine would have far fewer problems than my inline Lycomings...
Read up on the development of the early 190s (the As), might be an interesting read for you and the issue you're dealing with. The overheating problem with the early 190s was more of an issue with the design of the cowling and it's effect of air flow over the engine in combination with the engine's design itself, and less with altitude and thin air if I recall (though, obviously an issue to be considered). The BMW radials in 190s was a two-row radial engine design, meaning the first half of the engine's heads were arranged in a forward row, closest to the propeller and air intake. The second set of heads and back-half of the engine was located behind the first set of heads and incoming airflow for cooling. Also, in the first 190As, the engine was mounted/located as close to possible infront of the pilot and controls, making the overheating issue a particularly noticable and uncomforitable one to pilots.
AFAIK they never really adequatley resolved, completely, the issue of the rear-row of heads overheating (furthest from the prop and intake, closest row to the pilot) in the BMW radials and all the A-series 190s. It was a major issue in the first prototypes and first 190As, test pilots refered to flying the plane to like sitting with your feet in a fireplace. Eventualy (~ the time of the A-5 production) the first 190s had been around in some creative mechanics and crews hands long enough that some inovations and improvements made it back to the factory and into the production lines. The later 190 As eventualy featured longer cowlings and front ends (more space between the pilot and the rear heads, this also allowed more air circulation), larger cowlings with larger front openings (to allow more air in and around the heads), improvements to the cooling fan and gearing (the 190s featured a fan that helped with air circulation just beyond the propeller spinner and within the front opening on the cowl, see the OPs 2nd pic), and the iconic large "slots" for the exhaust and evacuating cooling airflow (a field modification on the first As that the factory then adopted as standard).
