There will never be a piston diesel engine that will have the power to weight ratio of a current helicopter turbine engine. Aviation diesel engines are only practical as an option for avgas burning piston engines.
Since the objective published 6 years ago was primarily to reduce emissions by improving specific fuel consumption, by burning to fuel at a much higher temperature, the investment should be in the greater use of ceramic materials in the combustion and turbine sections of all gas turbine engines.