I looked at the SMA turbodiesel having run a TDI in my VW Passat (only part of that car that worked well). I really liked the idea and when it came time to overhaul the O-470R, I looked long and hard at OSH. When the STC installed price came in at more than $70,000 (roughly twice what I paid for the airplane and 4 times the cost of a new limit overhaul), the engine was overhauled. Another issue, at least then was there was talk of a service ceiling limit of around 10k, which didn’t give me enough clearance to get over even the lower passes where I fly and certainly not enough to get in/out of high altitude fields I use, including LXV.
The mogas STC lives on, and I cart gas to the airport, or land at airports that have mogas all the while badgering my local FBO to give us mogas.
I’d be happy with a compression fired engine, if the power-weight ration works for the airframe.