why on earth would one need a new harness?
What is the problem? or are you building a car from scratch?
If you have had a fire, you could still maybe splice in new wires. To remove an old loom and replace with new you'd have to pretty much undo the whole car!
anyway, cable harnesses/looms came in various parts for the later cars, there wasn't a single loom like on old 60s alfas.
so you'd need to find the ECU/Injection loom, the instrument cluster loom, the centre console loom, and the main loom (contains fuse box and battery main leads and all the accessories etc)
Here, for example, is an ecu loom NOS in Germany for an Ljet S3 (basically ECU plug, injector plugs, fuel fuse, relay connectors, engine sensors, AFM etc)...... but, as you can see, it is not for free
Original Kabelbaum Alfa Romeo Spider 86-89 Serie 3. Gummilippe Einspritzer Neu | eBay