You can use either a deep/desk/dewy ecu from a mondeo with a fiesta Si loom but remember these are designed for a 2l, the best ecu for a 1.8 is the cult from a rs1800, this can be used on the Si loom, these are external pats and internal edis.

Other option is an early rs1800 or 130 spec xr3i which use ecu code 2fbc, this has no pats and external edis so you would need the correct loom for this ecu and also the edis4 box which plugs into the loom.