There are already alternative engines in wide use (particularly on ultralights and light-sport aircraft) that can run on regular, unleaded gasoline, like the Rotax 912[0]. The issue, as others have pointed out, is getting these newer engines certified for use on existing, older planes. This would require extensive testing to get a Supplemental Type Certificate (“STC”) to allow the engine to be installed on a certified aircraft (think anything manufactured, like a Cessna 172 or Piper Cub.)

0 - https://en.wikipedia.org/wiki/Rotax_912