Here is a 3d model air cooled engine with internal moving parts, pistons, piston rods, pulleys, crank and a cooling fan.
Read more for more information and the links to the basic version engine STL file and the purchase page of a improved version with STLs for every part of the engine.
You can get the standard version from my thingiverse page.
Or you can purchase the improved version with separate STL files for every part of the engine by using this link
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