The cetane issue is a mainly theoretical one. All major piston diesels (Thielert, Austro, SMA) are certified for the common avtur types. In fact, Austro and IIRC SMA aren't even certified for EN590 diesel. The engines appear to make TBO/TBR without major problems.
In fact, Thielert and Austro sell to the UAV world, it's basically what has kept Thielert afloat for the last few years.
Car engines are not ideal for airplanes but they do work quite well. The main issue is long term availability of parts. By the time they get to certification, the car maker has usually moved on to the next generation. Austro is supposed to have spent €45m in adapting the Mercedes engine -- the same model that Thielert used and with fewer modifications, i.e. less weight reduction. SMA have built a very conventional diesel with only custom parts, the result is not that great and the price is insane.
I'd say either car derived engines or a true aero diesel based on the
Junkers Jumo 205 design. Deltahawk appear to be closest.