Don't blag the old aero engines too readily. Efficiency is the key, and that is measured by how much fuel you have to burn per hour to produce each horsepower. All the gizmos on our car engines today are mostly for pollution control, and they would be hard pressed to better a Continental or Lycoming in the fuel efficiency stakes.
The Airworthiness Standards for aircraft engines are here
Electronic Code of Federal Regulations:
Of particular note are the testing requirements, FAR33.41 to 33.57. You are talking millions of dollars, and then some.