The number one place most of these old diesels develop an oil leak is at the botton of the air cleaner. There is a short tube on the underside of the cleaner and it has an O ring there. Over time the O ring wears and oil leaks around that oil trap. It is supposed to trap the blow by and send it down and back into the pan.
Unbolt the air cleaner and replace the O ring first. Cheap repair. It could be coming from a few other places. Clean up that oil and grime and try to trace the leak.