Cooling on the ground is going to be very different than the cooling that cowled engine will see in the air.

And unless you can record the head temps in the air, whatever temp is still in the head after you land isn't going to tell you anything. Unless you land with the engine running flat out and get to the site of the landing VERY quickly.

Does the engine sag in the air?
There is little reason a 91 on a 7.25pound airplane would get hot, unless you're trying to hover or harrier or such.

If the 91 is mounted at 9o'clock, try closing off the cowl opening on the 3o'clock side. Or put a diverter baffle in it like this one in my Decathlon. (I don't happen to have a clear picture of the same diverter in my Skybolt or I'd have attached it.)

Overheating is very often lack of hot air exhaust area or exhaust location. Next most is allowing cool air a path that skirts the engine on it's way to the exhaust opening. It will simply block the hot air around the engine from going anywhere. And the engine cooks.