Hi, virgin poster here looking for help already!

I have an 97 pre obd2 fiesta 1.25 and a 2000 puma 1.7 and want to do the conversion over a weekend, I figure ive a problem as I presume ive a 64 pin ecu whereas the puma has 104pin, the conversions well documented as is but ive only seen later 98 on fiestas getting it done. The only visual clue is the instrument panel has a digital odometer on the later models
So instead of an engine bay loom, ecu,key reader and key to swop will I have to change my cars entire wiring loom to upgrade to obd2 ? or will the dash and engine bay side part from the rest?