A lot of action is taking place to find 100LL replacement for GA aircrafts. This includes EPA testing four new unleaded fuel blends and some aircraft manufacturers installing compression ignition engines that will use Jet A fuel.
In my opinion there is a simple alternative that will substantially reduce conversion cost & will result in lower operating costs when compared to above two alternatives.
Consider using LPG (liquid Propane). This fuel is abundantly available and is at half the price of Jet A fuel. All it takes is to convert fuel system on existing SI piston engine to LPG port injection and install LPG tanks. No change in engine mounts, no engine cooling radiator. Current dual magneto system is retained.
This will increase power of SI engine, Propane has Octane number higher than 100LL, it is always contained (no venting, no vapor loss), it is safer than gasoline & Jet A fuel, no fuel contamination. Conversion could be done at approx. $15,000. Ideally conversion should be done at overhauling time to save on cost & down time.
Per dollar liquid propane has much higher heating value (see chart).