This is an Alpha mode hot air engine. (Hot air engines are commonly called Stirling engines but this is not strictly correct in many cases including this one). An alpha mode engine doesn't have a displacer as such. It has two (theoretically) identical power cylinders at 90 degrees to each other. The engine is powered by the temperature difference between the two cylinders, either of which can be hot or cold relative to the other. I say theoretically because, in this case, the supporting parts are made of plastic and so need to be protected from the hot cylinder. If both cylinders were similarly protected then they would be interchangeable. However, the designer has chosen to protect one side only (for cost reasons presumably).
Most of the engine parts are 3D printed in plastic, with a few exceptions noted below. The two cylinders are glass syringes (still marked in ml!) with the ground glass plungers forming the pistons, connected to the crank shaft with plastic connecting rods. The bore and stroke are both 12mm. The needle ends of the two syringes are connected together by a length of silicone tubing. The cylinders are mounted on a 3D printed plinth, thermally isolated on the hot side by a large steel nut!
The crank shaft is made up from plastic except the crank pins which are metal. The flywheel is plastic with some extra mass provided around the circumference in the form of several metal nuts glued in position. The crank shaft is supported by two standard ball bearings which are themselves supported by a 3D printed bracket. The two support brackets are glued to a laminated base.
The engine is not supplied with a burner so I am using a small glass spirit burner of the type typically supplied with small hot air engines.