Like I say, it will depend on the donor.
you won’t need to piggy back as the 3i will be very basic and not intergrated into other systems. Running the donor in the 3i is very doable you can run them any ware, on a pallet or in a boat.
I would start with a Blacktop Zetec ecu and loom from a mk1 Focus.
they both have 60 pin ecu’s.
I would use the focus engine loom and strip out the bits you won’t need like Ac. It’s then a case of intergrating it into the car. Just the same as an aftermarket ecu but there is a bit more of it.