It's a long time since I was involved in engine design, and diesels have come a long way in the past few years.
My present 2L diesel car engine produces over twice the torque of the previous V6 petrol, and about 75% of the BHP. It weighs almost exactly the same.
Max RPM is much lower - which is ideal for aero use. It achieves 42mpg average where the V6 did around 27 mpg - both the way I drive them.
Diesels also produce far less "waste" heat than petrol, so I don't see why a purpose-designed diesel for aero use couldn't be air-cooled.
The problem is that the sales volume would never justify the R&D it would take - I know what it cost to develop one all-new car diesel. It would scare you.