Ideally I would think another ECU with SLUG as they should be like for like. Fiesta RS1800 I believe ran 130bhp and opposed to the Escort GTI's 1.8 being 115 bhp. This will mean you may have some running problems and may need to have things mapped to get back a smooth ride.
I may be wrong but it may also not work with your PATS but that's just a guess!