Front brakes can be taken from any Sierra, and hence you have the option of solid or ventilated disks, and matching calipers. Ventilated disks come in both 240mm & 260mm diameter.  I have gone the 240mm ventilated route, although this is probably overkill on a car as light as the Mojo.

Rear brakes use the solid front disks from a Mk2 Fiesta 1.3 (not an XR2, which has vented disks) paired up with rear calipers from a Sierra or Granada.  Ensure you source the caliper bracket (which needs to be modified) as well as the caliper itself.

Donor calipers can be exchanged for reconditioned items at most motor factors.  I have also heard good reports for Brakes International. Note that it is normal to exchange the caliper only- retain the mounting bracket to re-fit.  My set of 4 recon calipers cost ~£150 from a local factor.

As I exchanged my calipers, I only had to clean up the caliper brackets.  This is best done with a wire brush attachment fitted to an electric drill.

The rear caliper bracket needs modifying as per the online build manual, although the use of an angle grinder is recommended.  When modified, the caliper bracket should be perfectly centred on the brake disk.  Once this is done, a coat of Hammerite will help to stop the brackets deteriorating in future.

It is also common practice to fit new disks and pads, as they are relatively cheap.