The most promising short-term approach is switching from spark ignition to compression ignition: Jet fuel is very comparable to diesel fuel, and doesn't have the same problems with lead that avgas has. It has a higher energy density, providing lower fuel burn and longer range. And it is significantly cheaper than 100LL, let alone 100UL.

Europe has a significant number of diesel-engined general aviation aircraft burning Jet-A. Several manufacturers have received supplementary type certificates to swap out gasoline engines for diesel in many of the most popular GA aircraft.

Longer term, electric and hydrogen are being researched, but it will be a long time before they are viable options in the GA fleet.