I've been thinking to make a computer case to look like a 4 cylinder engine like this.http://www.mofocases.com/cases.php?case_id=8
That link is to a V8 but a inline 4 cylinder would be idea for a single computer case.
I would have more detail like make the power supply look a distributor and have the cables from the power supply look like spark plug wires going though the valve cover into the case. For the intake manifold It could be a water tank for a water cooler. Use the same spots where the PCV valves go for the water line for the CPU and have a ram air intake on the manifold to house the radiator for the water cooler. Put the water pump for the cooler where the water pump of a car would be located. Take two 80mm fans and cut off the blades and use a belt to move around. Maybe 3 fans and make a fake alternator. Also I can make it look like it's carb instead of EFI and have the radiator to look like the carburetor.
So many ideas to make a computer case to look like a engine.