The requirement to work on "N" registered A/C is as far as I am aware not relevent nowadays I know of several People who took it and did not need to show such requirement when I took(1992) mine all you had to do was have your experience endorsed by the CAA which was a bit of a pain but I believe they will do it these days on payment of the appropriate fee (surprise!!) when you get that you go to the states have an interview with an FAA surveyor and if you meet the relevent criteria you will be allowed to sit the necessary exams, good luck.