However, tetra-ethyl-lead (think one lead atom bonded to four ethanols minus the oxygen atoms) was never necessary to combat the problem. Ethanol's anti-knock properties at a blend of 85% gasoline 15% ethanol were well known, and issues with corrosion in fuel lines had been dealt with. However, that meant giving farmers (the only ethanol producers) 15% of the profits, and the famously monopolistic Rockefeller didn't like that. Indeed, about that whole Rockefeller-financed ethanol temperance movement, and Prohibition in general...
In addition, alkylation strategies in World War II by Shell produced 100-octane aviation gas for prop engines, with no lead. It's been reintroduced it seems (wiki):
Shell Unleaded 100-Octane Fuel
"In December 2013 Shell Oil announced that they had developed an unleaded 100 octane fuel and will submit it for FAA testing with certification expected within two to three years. The fuel is alkylate-based with an additive package of aromatics. No information has yet been published in its performance, producibility or price. Industry analysts have indicated that it will likely cost as much as or more than existing 100LL."
In the long run though, electric airplanes look far more attractive for the short-haul prop-driven world.