Another alternative would be coming up with some organometallic that replicates the properties of TEL, the transition metal(s) of which are something that's not lead. Or mercury, uranium, cadmium, and a few others that are really easy to avoid. Materials chemistry has gotten lots better over the past hundred years, the meaningful development costs are only in validation testing. The FAA "should" be able to cover that cost instead of individual pilots converting their engines.
