The Clio II immobilser code is alocated to the vehicle for life & any new key or UCH has to be coded using a Hexi decimal code from Renault net, the injection computer also retains the code for life but if replaced with a new unit that new unit will receive the code from the UCH & retain that code.
If you are changing the injection ECU for a pre used item you will also need the UCH & key as you have,
PLUS the original chassis number from the donor vehicle for future repairs as Clip cannot re-alocate a immobilser code from the original vehicle to pre used parts.
There are companys like Fastchip in Holland who might be able to reset the injection ECU to a blank immobilser code so you could fit the modified unit to your vehicle so you will keep the factory code.
fastchip
Many a time I have informed customers that they need a complete new system because they have fitted a used set & never recorded the donor vehicles details or tried to fit single items & wondered why the car will not start.
Let me know if you require more info