A big hurdle that has yet to be discussed here and can be hugely expensive to address is "torsional vibration".
An example - look at the difference in TBO and finite life of the TR gears in a 300C versus 500C. Exactly the same gearbox but massively lower lives when fitted to the piston machine.
Avgas burns slower than Jet or Diesel in a piston engine and the power spikes are lower and longer. With a diesel this while be a real problem to damp without introducing mass of some sort.
I always find it amusing when people think they can just put a car engine in an aeroplane and away you go. Has never really happened yet has it? Wonder why? The operating regimes are miles apart.