Personally I'd would not allow the used engine - you bought a new car with a warranty & they need to repair it properly - sounds to me like they are trying to pull a fast one. By the way by best friend is a service writer for Ford & i asked him & he said they would never go the used motor route & couldn't believe it.

 

They need to either rebuild your engine or put in a new engine. Don't agree for them to swap in a used engine that they "say" has 5,000 miles on it