If your wanting cheap running, have you thought about a Diesel ?
Lower fuel cost as it uses stock JETA1, plus about a quarter more range on the same mass of fuel compared to AVGAS. Simpler operation, no ignition system (less Electromagnetic Noise). Plenty of companies in Europe pushing this technology at the moment and with the FADEC stuff bolted on its almost leading edge. Well, 28% MAC maybe...
When thinking about auto-conversions you'll always come up the following argument.
The majority of aviation (reciprocating) engines are made to run for extended periods of time at a relatively low rpm (say 2700).
To get the best out of a automobile engine you'll need to rev the guts out of it, say (6000rpm or higher). Would you be prepared to sit behind a car engine running at that speed for a couple of hours at 3000'. Try it in your own car, just jack the wheels up
Moreover, the really clever bit for auto conversions is going to be the gearbox bolted to the front of the engine. You'll need to get that rpm down to a manageable prop speed. Another place where any problem will be mission critical.
Hey, no-one ever said it was going to be easy. Best of luck with your design. Pity you can't call it the ETOPS773 - Boeing might have something to say....
Father Mulcahy