An engine is not just cylinders, pistons, crankcase, and crankshaft.
There’s every component of the valvetrain, every component of the oil system, every component of the accessory drive, every component of the magnetos, every component of the starter, every component of the alternator, every component of the injectors or carb (hundreds of parts on its own), seals, gaskets, bolts, nuts, washers, studs, shafts, pins, bearings, wires, connectors, sensors, covers, clamps, etc. On an IO-540, there are 22 parts on the generator mount alone, 86 parts on the fuel lines, 257 parts on the oil sump and induction housing.