Before you look into the many auto and motorcycle conversions, with
their "shell game" of horsepower claims, take a look into a couple of
certified aircraft engines you can buy for similar costs, with much
greater simplicity and ease of installation. I'm assuming you're in
Europe, so my two choices there would be a)PZL Franklin O-235, and
b)Moravia LOM inline, (I forget the designation.) Both engines have
been around forever, and you can buy a factory remanufactured engine
in either one for under 10K, unless prices have gone up, lately. And
both engines have accessories, engine mounts, etc., that were
designed, or can be easily fabricated/duplicated from a certified
aircraft.