This discussion nudges closely upon the Mogas discussion. There are two camps with the Mogas idscussion, those who think it's great, and those who either cannot, or will not use it for their own reasons.
In respect of certified aircraft, any fuel used must be certified in that aircraft and engine. For gasoline powered aircraft, this it typically Avgas, whose most common North American standard is ASTM D910. When the US EPA bans 100LL, one of two things will have to happen:
Either the replacement gasoline (whatever it is) will have to conform to ASTM D910, in respect of those aircraft which specify it as the only eligible fuel. This could be accoplished with changes to the standard for D910, but that does not happen fast, or without a lot of testing. The new fuel will also have to conform to the Type Certificate Data Sheet specifications for the engine and airplane, or each manufacturer will have to change every TCDS. Or,
Each subject aircraft and engine is individually approved to run on the new fuel by STC. This will be an immense task, with people expecting to be paid for that work, and make a profit sufficient to cover the inevitable liability, for those who have problems flying with the new fuel.
Either way there is cost. A number of Mogas STCs were developed in times past, and worked excellently on some aircraft. Other aircraft and engines were never STC'd, for the most part becasue Mogas just does not work safely in those types for various reasons.
The fundamental differences between Avgas and Mogas result in some aircraft being entirely unable to run on Mogas, or any gasoline with similar properties. The major of these characteristics are "Octane", which to some degree can be worked around, and volitility, which if not compatible, is near impossible to safely change for. The compatability of the aircraft fuel system is another issue not to be overlooked.
We are in for big changes in respect of fuel, in the decades to come. I opine that diesel is the way of the future, so everything flies on jet fuel. Yes, there'll be Avgas for a while, but knowing it is going away at some point, does not make a 100LL dependant aircraft a good long term investment.