The engine bay loom for the fiesta and efi 3i are essentially the same, there is a yellow plug on most escort looms which has the power for an engine bay loom.

the bay is a standalone loom essentially, it has a couple wires which connect it to the car, like power, earth and rev counter signal etc, from there it does its own thing. But if you use an xr3i loom you'll need to change some pins around to plug in the OFAM. Unless they do it on the ecu when building it.

Therefore you don't specifically need a certain loom, which makes finding one a bit easier. That was my point.

As Karlos says, good idea to do continuity check on the loom anyway, you could also test resistance for the important wires cause they may have degraded giving more resistance which would cause false signal.