Perhaps if Rod wants simple, reliable, and light, he should talk to D-Motor. They have a 4-cylinder 2.7l and a 6-cylinder 4.0l flat-head engine, with nicasil barrels, impossible for valves to ruin an engine, direct drive, water cooled and same mount as 2200/3300 engines. I'm sure he will be reminded of the KFM adventure, but by making proper arrangements regarding IP, Jabiru can get themselves a new engine that is way beyond the prototype state, and without the finnicky air cooling issues.
I believe there's actually a Jabiru airframe already equipped with a D-Motor