How can i fit a Citroen 1.9D engine into my Peugeot 406 1.9DT?

HI, I have a peugeot 406 1.9DT 96/P-REG model. recently my cambelt failed so i have unfortunatly bought another engine. I was looking on ebay and came accross a citroen 1.9D 93/L-REG engine and was told that it would fit into my peugeot 406. I have now bought the engine but my mechanic is worried that the ECU may not recognise the engine and won`t work.
I NEED TO KNOW WHAT DIFFCULTIES I MAY HAVE OF FITTING IN THE ENGINE, CAN SOMEONE PLEASE HELP ME!!!