Was the MK1 with the 1.6 litre engine sold with SOLID disks in the USA?
Standard on MK1 and MK2 XR2 cars were 239 mm vented disks and there's still a lot of fast road/sports/racing pads available. And I'm quite sure there's still grooved/drilled disks available in the UK.
Fitting bigger brakes (with "standard" Ford parts) won't work with 13" wheels. Wilwood had 4-pot-brakes for the MK1/2 Fiesta but I'm not sure if these are still available.
And no, other brakes (Escort MK3/4, Fiesta MK3 etc.) won't fit as the mounting points for the brake carriers are different on the MK1/2 fiesta.