As a prospective student pilot in Southern California, I would like to avoid 100LL. Since the FAA 100LL phase-out is stalled, what other alternatives are there? Continental has a line of Diesel engines such as the CD-135, are supplemental type certificates available / retrofits common? What about "mogas?" Does the answer change if someone is looking to purchase an aircraft?
Right now, Mogas is the only alternative fuel because the unleaded avgas program still seems to be going nowhere, although the industry testing program is supposed to restart this year.
You could put an aircraft diesel engine in your plane if you're rich, and run it on Jet A, but if you can afford that, you can probably afford other alternatives as well.
The most practical thing for a prospective owner to do is to buy an airplane and incorporate the Petersen Mogas STC (it's just a paperwork/placard mod for a lot of high wing airplanes, but physical parts may be required in other cases), and run the plane on mogas when possible, and avgas when not.
Overall, Mogas is better, because the tetraethyl lead in avgas used to get its octane rating is just bad news all around (the lead combustion byproducts contaminate the oil, leave crud in the cylinders, etc.) Mogas's main caveats are higher Reid Vapor Pressure (it boils more readily) and long term storage. Mogas is more volatile and needs fuel stabilizers if a fuel supply is left for more than few months. The volatility also means more susceptibility to vapour lock.
On airplanes with gravity feed fuel systems, vapour lock is less of a problem because there is positive fuel head pressure all the way from the tank to carb. On low-wing pumped-fuel airplanes, there is pump inlet suction, which to the gas is like going up a several thousand feet depending on how hard the pump sucks, so on a very hot day with a hot engine, there is a chance of vapour lock and it can be prudent to install cooling shrouds on fuel system components like engine driven pumps (you'll know it's happening as the fuel pressure fluctuates). I was having incipient vapour lock indications on my plane during heat waves (only on the ground) and installing a cooling shroud for my engine-driven fuel pump seems to have solved it.
Those issues aside, any 80 octane engine will be MUCH happier running on Mogas than 100LL, so if you can do it, it's a no-brainer.