youve got other issues to consider, the pats system and alarm system changed on later mk1 focus as it did on late pumas so you need to match up the right age of ecu to your car.
Also be aware that your dashboard is likely to take its speedo and rpm feed direct from the ECU via a databus which early puma ecus dont support. Later pumas can do it in theory as the connections are physically there you just have to hope theres no software issue restricting databus communication to diagnostic equipment only!