You can use the same AC compressor; you just have to remove and rotate the clutch 90 degrees. However, I find it best to use a new hose assembly to the compressor.
You will need two keys from the old PCM, if you want to program the keys that fit your existing ignition yourself.
If you use your SPI transmission, you will have to have the SPI gear ratio programmed into the Zetec PCM.
You will also need the coolant hoses and the little pipe that mounts above the radiator.
A new power steering pump and line to the rack is required.
The Zetec also requires a different cat converter & flex pipe.
You will also need a new throttle cable and the positive battery cable to the starter, which goes on to the alternator on the Zetec.
You mentioned the main harness, but you also need the engine harness and the transmission harness. The engine harness may have come with the Zetec, but you will find that the transmission harness between a SPI & Zetec are different (i.e. crankshaft position sensor, O2 sensors, etc.).
The vacuum line to the power brakes is also different.
You will need the small vacuum lines that connect to the intake manifold.
Also the fuel line to rail hose assembly is different.
You will need a new alternator.
In additon to the passenger side motor mount, you also need the bracket that it bolts to.
That's all I can think of for now. I've done 3 of these and have parts in the garage for 2 more. Let me know if you run into any problems.