It all depends on the registration of the aircraft and the ICAO (I think) agreement. I can fly single engine Day VFR in the UK, if I wanted to get a CAA PPL, I would have to take 3 exams one to get my radio operators license the other on air law. I reckon when you get past you commercial the restirications get tougher. I know if you have a JAA ATPL and want to convert it to an FAA ATP, you will have a to all the writtens and the checkrides above your PPL. If you do it the other way you have to a 6 month course (if you have over 800 TT), and take 13 writtens.
You really need to look into the companies that operate down there (see if they are FAA) and then look into converting. I reckon from Canada to the US it would be the writtens and a checkrides for IFR, ME, COM, ATP, but you might be able to do some in the same day/same flight.