I don’t think there are enough sales of new GA planes in the US to fund the r&d of a small Diesel engines, much less multiple competitors in the same market. The cost to replace an existing engine in an older would be astronomical, so owners aren’t going to do that until avgas is totally unavailable.
I think there might be a chance to follow the Rotax model and start in the experimental and UAS markets and work up from there. Maybe Cirrus’s new trainer will be a good platform for a diesel power plant for the overseas market. It hasn’t worked well for either Cessna or Piper, but they don’t innovate well in the low end of their fleets.