It's good but not quite there
I have thought about this quite a bit. I have a 2002 Boxster that I thought would make an excellent aircraft engine. It is a flat 6 with 350 HP and no fan belts to break. On the move to TX from CA I drove it continually for 10hrs a day at highway speeds (okay, a little over the posted limit). No issues and it had better gas mileage when running at those speeds.
BUT, I have had a few issues come up that I would never want to see in the air. (valve broke and the intake ingested oil) Also the gearbox can?t really handle the power. And this is ?German engineered?. There have always been problems the mating of the engine and the gear box. It?s power band is somewhere between 4.5K and 5.5K, so not really prop speeds. And it ain?t cheap to keep running.
Although I would consider it almost bullet proof, I would not want to fly behind it.
I love the guys who live to tinker. I am one of them, but not on engines. I would love to see someone build a aircraft engine with all the modern efficiencies (easy start, computer controlled, electric mags) But I guess we can get that now in a Lyco.