Fiesta 1400 down to 1.25

I need a new engine for a R reg '98 fiesta 1400 zetec and am having difficulty in find one, has anyone replaced one with a 1.25 as they look identical. My main concern is the engine management side.

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FITTING THE ENGINE. IT WILL GO STRAIGHT IN. BUT AS U SAY THE ENGINE MANAGEMENT WILL MESS AROUND AS IT IS SET UP FOR A 1.4 U WILL BE PUTTING A 1.25 INTO IT SO THE FUEL PRESSURE WILL BE WRONG AN THE AIR PRESSURE WILL BE INCORRECT!!!! CAUSIN IT TO RUN LIKE POO BASICALLY.
BUT HOWEVER I HAVE KNOWN CASES OF THEM STARTIN AN RUNNIN FINE BUT BECOMING WORSE!!!!
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