There are basically 2 approaches to eliminating leaded AVGAS.
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Formulate a new blend that has the same performance characteristics as 100 LL, or
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Replace 100LL with an existing unleaded certified aviation gasoline, namely 94UL, which is basically 100LL without the lead.
Choice one has proven to be unexpectedly difficult because there are no easy and cheap chemicals which can replace TEL. The various formulations of 100 UL require quite expensive exotic chemical additives which bring the significant potential for unintended consequences to either airplane fuel systems, or the environment, or both.
Personally I think 94UL is the solution. It involves zero change to the existing aviation gasoline refining and distribution process and is price neutral and a reasonably long phase in period can be announced now. The downside is, of course, what about all the engines that are only certified for 100LL.
Well… and I know I am going to get hate mail for this; but I think those airplanes are going to have to be modified to run on 94UL, which for most will mean a set of lower compression pistons and a slight reduction in takeoff power output. Turbocharged engines will also probably need a water methanol injection system. Of note Continental already has a version of the turbocharged TSIO 550 which is approved to run on 94UL.
However I think it is likely that many engines can probably be run on 94UL with no changes because there was no reason to ask the question. An example is the IO360 in the new C 172’s. It was a 100LL only engine but is now also certified for 94UL with no changes.
I would suggest that this is where the FAA could be part of the solution by providing a simplified recertification pathway for approving 94UL in existing engines and where that is not possible a simple approval for de-rating engines with an AML equivalent. They also could provide a subsidy like the ADS-B program.
The bottom line is GA is going to be paying more going forward. We either pay a substantial penalty per gallon for 100UL forever, or an upfront penalty to modify engines to use 94UL. Pay now or pay later but you are going to pay…