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Desperately looking for a title
You can import the engine as parts, but it is a Federal crime to install a diesel engine in a gasoline car if the car was not approved for sale here with a diesel.
So, you can convert a W126 380SE to use the current Merc BlueTec diesel since the W126 was approved with a diesel motor back when. But since the Audi 80 was never approved for sale here with a diesel, you can only ever do gasoline engine swaps, legally.
Guess who was responsible for this.
And of course everytime someone tries a petition or something nowadays, every two years or so actually, you realize just how tiny the number of people who actually give a crap are.