Onetrack makes many good points...
But meanwhile some people just quietly get on with viable and simple diesel motors that seem to work really well. The Wilksch is simple and apparently reliable (so long as the installation and cooling requirements are sorted properly).
But the French homebuilt scene is thriving on PSA/Citroen car engine conversions. The successful formula is to build ultra lightweight airframes and use a simple belt drive reduction gear on the lightened car engine.
No homebuilder uses common rail because it is inherently unreliable, as a simple electrical failure will mean loss of the engine.
All the successful engines so far use tried and tested super reliable mechanical injection. Both the Dieselis and Gazaile achieve greater than 80 mpg and endurance of nearly 10 hours. Maybe it's as always the cost of certification is the main obstacle to diesel development for GA.