There isn’t a step by step for this, unfortunately you need to churn through the two wiring diagrams of the two cars you have to get what you need. It is part of why it is so important to know what year your 128 really is as the wiring can be vastly different based on the year of the car during the years this car was produced massive changes took place in regards to safety and emissions systems so there were big differences between some years.
I would read back through the front drive part of the forum and become a member of MiraFiori as some of the old time members there did have 128s and may be able to help you but remember those cars and those members likely used carbs in hopping up their cars so don’t expect them to give much input on fuel injection.
We spoke about just putting the carb and it’s manifold into the 128 with the 1500 to get the car on the road and fix the other things this car will inevitably need. Putting the injection system into the car will be a bigger, fussy job and given the quality of the wiring I see, you likely have wiring repairs to make for it work reliably.
I think the following two posts by me cover much of what you have asked for:
Just picked up a 73 fiat 128. Debating to swap engine or rebuild original. It has a 1.3L 4 speed but I believe once rebuilt it won’t be fast enough to flow with traffic here in LA. 80mph average. I just don’t know what front wheel drive engine to drop in. I’d like to make it rear wheel drive but...
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Sorry to hear about the mismatched title. Time for decision making. I don't know how crazy your DMV gets with registering cars. Pretty simple in Virginia as they just look at the title. Also, at some point you will sell the car and you have to decide if you are going to be fully up front...
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You do need all of the wiring in the X’s spare tire well. There isn’t much of the rest of the wiring harness you need from the X, so you won’t need to pull wires all the way back to the instrument panel (if your 128 doesn’t have a tach you may wish to add a wire but you shouldn’t need the exact wire from the X as on a 128 it is a much shorter wire). What you do need is the wiring from the fuse box to the double relay and then the wires from there to the various components and the ecu. All the ignition related wiring in the engine compartment, the wire from the ignition is the same so you won’t need to go back to the ignition switch.
You will want to ensure the 128 side of things remain 128 to make it easy to resolve wiring/mechanical issues and the fuel injection, electronic ignition wiring to remain as much like the X as possible so you can easily use the manuals and expertise for the fuel injection that does exist on the forum.
Looking forward to seeing things start and we can help you over the inevitable humps and bumps.