Factory may be a misnomer - Nissan North America Inc. will not support or warrant the vehicle. You will need to source parts elsewhere and insurance is a a bit more difficult. No worse than any nother low volume vehicle tho. Superformace S1, Caterhams, Sun Elises, all available. FOr the price of a Motrex tho, I'd prefer a Noble M12.
Motorex is a RI - Registered Importer; they post a serious bond, "federalize" the car - making sure the car is EPA, NHTSA and DOT compliant. They specialize in Skylines, but there are quite a number of other RIs around. Its an expensive process, but the only legal way to import a vehicle not otherwise obtainable legally. The way its regulated now, almost any RI is very reputable, unless they want to loose their ass. Motorex does a good job, but their product is pricey - no way around it. Well, legally. Vehicle 25 years and over are exempt, so they are importable easily. Other "loopholes" exist, but they mostly are not practical.
I'd imagine there are a great deal more JDM cars that are imported and reconstituted, or are morphed 240SXs, with varying degrees of legality - but really, how would they know. Unless you are in a major accident, when the adjuster/investigators will start asking questions, doesnt appear to be an issue - least from all the JDM vehicles I see running around. Little market in Japan for used cars, so it becomes econommic for them to go elsewhere. Many end up here, in whole or part.
Its not confined to Skylines, or even cars. Used to see a few RG500G, and still see things like BJ42 Cruisers, and late model Defenders, they just get driven across from Canada.
Luck
Alac