Naval systems are more complicated than land based ones, this is valid for the engines .a naval engine is bound to run for weeks without any interruptions, at spiking variable loads (propeller surfacing-sudden load drops, sudden increases due to harsh weathers) thus it is not just easy to make one, while we even couldn't succeed a tank engine.
It is not just the engine but you need to make sure of multiple aspects, you may not mariner hear people calling it "engine" since from oil separators, cooling system to the end of propeller it is referred as "propulsion system" which includes oil separators, fire suppression systems vibration /acoustic isolators, transmission system, shaft (with propeller blade controlling hydraulics), shaft bearings (vibration isolators), stern tube-thrust bearing and finally the pitch controllable propeller.
beside achieving all these, despite of people usually consider ships as a "spacious asset to fit any kind of the engine", actually it is quite narrow and it is sometimes hard to fit 2 diesel engines side by side.
It is more critical to keep hands on spare part production, engine control software (which i believe is developed or being developed) and small machinery which is needed urgently in case of failure or in wartimes.
Moreover, there aren't any issues or embargoes regarding to the diesel marine engines.
FACs will going to use, probably, entirely gas turbines (COGAG/COGOG) which we are not intended to develop in short term.
