I've not been involved in putting a 200TDI Disco into a Defender, so someone else that has, should chime in.
A good read: http://www.glencoyne.co.uk/tdiguide.htm
The important bit is this: "The Disco 200TDi uses the same radiator and intercooler as the Defender, but the frame is different.". Whoever did the swap should have moved over the radiator as well and simply modified the frame to suit. Maybe someone can help you find a used frame cheap to help.
Putting the oil cooler back in sure could not hurt.