Rebuilding old diesel engines (e.g. 240D)

[ericnguyen]

Dear friends and LarryBible (who rebuilt several diesel engines)

Is it true that rebuilding old diesel engines (e.g. a 240D engine)
is not possible because of very high cost for parts (e.g. piston rings etc.)? So rebuilding an old diesel engine these days means replacing it with a whole new one taken from another car (savaged)?

Is there a cheap way to cure an old diesel engine that smokes
without resorting to replacing it with another one?

Thank you very your information.

Best regards,

Eric