Does the X you have start and run now? I would work on getting the X running to ensure that moving all this stuff will be successful. Trying to figure out if the problem is the parts or the install when the car won’t run is worse than frustrating.
Download the manual for the X and the 128 from Mira Fiori
Down load this from Mira:
https://www.mirafiori.com/faq/fiatFI_may2002.pdf and get conversant with it, although it centers on the 124, the system is effectively identical in terms of what is there and how it works with the exception of the intake manifold coolant heating system which doesn’t exist on an X.
The fuel injection is effectively a stand alone subsystem added to a carbureted car. This image from MWB shows the primary components:
First get the X with FI wiring diagram, it will help you identify what to look for, preferably the exact year of the car, you may find it here in the WIKI or by becoming a member at MiraFiori.
Here is a video of some of the conversion and some of the parts:
All of the injection electronic hardware and most of the wiring either start or end in the X spare tire well. There is the ECU and the combo relay in there. Start with the combo relay, there is power at the fuse box with an inline fuse that provides power to the combo relay, pull that from there back to the combo relay. From the combo relay outward you will remove the wiring related to the fuel pump and over to the ECU.
From the ECU there is one large connector with a rubber seal inline which goes from the ECU to the engine bay on the other side. This leads to the ignition system and all the sensors on the engine along with air metering assembly etc.
You will want all the brackets and wiring related to the electronic ignition which has a relationship to the injection system as well. Again another basically stand alone system added to a car which was designed to have points.
I would disconnect the main harness from the ecu, push it through the hole and then drop the engine with all the components attached to the engine and the harness as a unit. If you choose to pull the wiring, label each thing with a painters tape tag, photograph the tag and the connector and what it is attached to and then take great notes.
There isn’t a complete guide, this would be your opportunity to create such a thing to guide others in how to do the conversion. Many people have made the conversion, they just weren’t the type of people to do a how to tutorial.