I recommend going or writing the FAA in the states. They will have a look at your logbook and see what gaps need to be filled in order to take the check-ride and exams. There are only two written exams and an oral/practical examination for each rating. Also try
www.ALLATPS.com, I've worked with them before and they seem very reasonable and quick. You could possibly do it all on a visitor's visa. Just have all of your ducks in a row before you head over. If you have any other questions PM me. I have converted several people from JAA to FAA.
Also, some of those N registered aircraft have solely JAA certified pilots flying. Thanks to some sneaky accountants.