I'll get to reading everything as soon as possible.
Is a liquid cooled engine an option? I'd love to use a boxer from a Subaru, but it seems that they are used for bigger airplanes.
Edit:
I am talking about an aircraft about the size of a Jodel D11. That might change, but gathering info can't hurt.
Yes, liquid cooled engines are used successfully on many aircraft. In general, to take advantage of the greater HP per cc liquid cooled engines are capable of, you must run them at higher RPMs than a direct drive prop can accommodate ( the prop tips shouldn't exceed about 0.8 mach). So now you need gearing to slow down the prop RPMs relative to the engine RPMs. These Prop Speed Reduction Units (PSRU) add weight and about 1000-2000 euro to the cost. And you'll need a radiator, which also adds weight.
For these reasons, in the 60-80 hp cheap engine market, VWs are very popular. They are relatively light, they are relatively cheap, they don't need a radiator or PSRU, parts (including parts needed to make big ones) are widely available and cheap in many places, and they are simple and easy to worlk on. They have been used on airplanes for at least 6 decades, and the path to a successful conversion is well known. If you have more money to spend, then Rotax 4 stroke can work. In Europe, BMW R1100 to 1150 motorcycle engine conversions are popular. But, again, you'll want to use an engine that is known to work in the plane you choose, with a well supported path to success. Don't let the engine "tail' wag the airplane " dog".
Check into the Jodel D18 and D19. It uses a Jabiru 2200 engine which puts out 85 hp. It is a little stronger and a little lighter than the 2180 cc VW engines (which you can build from parts). Maybe some builders are using the large VW engines successfully, I don't know. The D18 and D19 will accommodate 2 people and is reportedly, like all Jodels, a very good airplane.