Certifications work on a per-aircraft-type and per-engine-type basis. Converting a single aircraft type to a different engine is possible, but every single comb

Certifications work on a per-aircraft-type and per-engine-type basis. Converting a single aircraft type to a different engine is possible, but every single combination would need to be tested (I think this also includes flight testing) and certified individually. This is not economic. So what remains is for the existing fleet to eventually be retired and replaced by newer aircraft that use newer engines. But most light single piston aircraft being sold today still use older engine designs that require leaded fuel! And newer aircraft are so expensive, it's more economical to keep aircraft from the seventies still going. Newer aircraft are expensive because the certification and insurance requirements are extremely expensive. For example, aircraft manufacturers get sued for accidents caused by pilot error on a regular basis, and so they need insurance for that. "Average cost of manufacturer's liability insurance for each airplane manufactured in the U.S. had risen from approximately $50 per plane in 1962 to $100,000 per plane in 1988, according to a report cited by the Bureau of Labor Statistics,[14] a 2,000-fold increase in 24 years." --https://en.wikipedia.org/wiki/General_Aviation_Revitalization_Act https://en.wikipedia.org/wiki/General_Aviation_Revitalizatio... So light aircraft are stuck running ancient engine designs that require leaded fuel. There is some progress. One manufacturer uses a modern engine that runs on jet fuel (basically a diesel piston engine like in some cars). But that doesn't stop new aircraft being extremely expensive, so it looks like we're still stuck with mostly an old fleet that uses lead for a long time yet.