I have done some reading into kitplanes and engine alternatives. Since kitplanes are considered experimental, you don't have to use a standard aircraft engine. This means you can save money on fuel and maintenance. Initial cost also seems to be less for these alternative engines. Here are two I have found and think they are interesting.
Like traditional, 20th century aircraft engines, the boxer Eggenfellner engine is horizontally opposed. But it’s similarity to old-fashioned engines ends there. Because this engine isn’t air cooled. It’s liquid cooled. And that makes a big, big difference.
DeltaHawk's Aircraft V-4 Turbo Diesel Engine is designed from the ground up as a compression-ignition general aviation engine designed for simplicity, durability, fuel efficiency and power configured to accept a fixed or constant speed propeller. [It is also a] springboard for other applications requiring light weight compression-ignition engines