Technically, they wouldn't need FAA approval, but they would need CAA (and probably EASA) approval. If it was something that had been previously approved by the FAA (such as the 747 3 engine ferry) getting such approval would be trivial. But if it's something that's not been previously approved, the local authorities will want to know why, and what could go wrong (which in the case of a single engine ferry is painfully obvious).