If your engine has the original oil filter housing, there should be fittings to attach to an oil cooler. Here is a picture of a turbo 603. The rigid lines from the oil filter housing go to the oil cooler ahead of the left front wheel -



You should be able to swap in an oil filter housing from a US spec 601/2/3 or tap your oil filter housing if it isn't meant for an external cooler. An option is the oil-to-coolant cooler from a 95-97 US spec naturally aspirated 606.91x. I think it sits below and ahead of the oil cooler next to the oil pan.

Sixto
87 300D