You would be mad to buy an N reg plane, re-register it on the G reg and then leave it in the USA......absolutely barking mad......Why would you want to do this? If it is for licencing issues, it is a piece of cake to convert to the FAA.....or do you want to do something a bit more sinister, like JAA Flight Training?
Doubtful you would find any EASA approved maintenance organisations in the USA (other than BA perhaps)