There are currently no Diesel/Jet-A1 aero engines in that range that I am aware of.
The Austro Engine / Steyr 6-cylinder in-line engine should be much closer to certification and with Diamond and the experience with the AE300 behind this, I would give them a much higher chance to succeed in the conventional engine field. While the AE300 is based on a Mercedes car engine with questionable end of life, the AE500 should be able to address that concern.
The SMA/Continental is rated at 230bhp and I would expect them to come up with a 6-cylinder design long time before that new venture even starts test flying their design.
If I was to construct a new aero diesel that is not based on an existing engine, I would neither use a V-type engine, nor liquid cooling nor a reduction gear nor 4-stroke...
With Thielert/Austro on the conventional car side, SMA with a 4-stroke aero design and Deltahawk on a novel 2-stroke concept, it doesn't look like an attractive market for a new venture.