Not sure about America, but in Canada, I was told by some dealership (a Subaru Dealership as I was trying to import a WRX engine for winter kicks) that it's not possible to import a whole and new engine. However, you could import a USED engine and have the engine taken apart into 3 or more pieces and consider those as parts.
Not sure if you wanna do that to the heart and soul of an automobile though....