You can't put a car engine in a plane
I've been flying planes a lot lately. For planes, the biggest issue is regulation of approved fuels and engines for a particular aircraft. Currently, aviation sticks to Jet A or AVgas engines and they are defined as piston or turbine engines with strict guidelines for certification. If you have an experimental aircraft, you can get away with more, but can't fly it in the same places or over populated areas. Until the FAA grants non AVgas engines clearance, there aren't many options.
This company is making a conversion to run off flex fuel, a lighter aluminum chassis, and AVgas too, but I have no idea what they'll have to do for the FAA to approve their design.
OUR MISSION Manufacture modern, reliable replacement engines that reignite general aviation and reduce our environmental footprint. 11/2022: C172 Experimental Update […]
corsairpower.com
You can use just about any engine in a boat. Inboards are basically car engines. The catch is that they often are coated with marine grade paints, grease, and sealants. Most ocean-going vessels are powered by diesel engines. I know quite a few sailboats I've seen had smaller tractor engines due to their 20-30hp, compact size, and superior fuel economy. Of course, if you want speed....you can easily get a 300-500hp engine. It just takes a lot of balancing power along with prop size, pitch angle, boat weight, etc....to avoid prop cavitation and general poor performance. Point is, there's less to screw up on a boat than an aircraft, but still can be a lot to it.