We are on leaded fuel precisely because FAA makes it too hard to certify new GA engines. Private companies have every incentive to use better, cheaper and more reliable tech, and consumers want them too. Without FAA, we’d long be flying on automotive diesel engines.
It does exist, it has been approved by the FAA, and you can go buy the STC right now if you want[0]. The incentive is that 100LL has been banned by at least one county[1] with states[2] trying to follow (although the AOPA successfully neutered that one[3]).
They don't need to develop an alternative to 100LL. Oil companies make it in such small quantities it's a total pain in the arse to cope with.
If you adapt a car engine (like a VW Beetle or Subaru engine, both flat-fours and common lightweight GA power plants) then it just runs on ordinary 95 octane unleaded, the exact same stuff you put in your car.
