I would need to know the Mod state of the radio kit that is fitted but off the top of my head if you have a King KX155/165 fitted that is post MOD 13 then it is FM immune.
Painting an "N" on the side won't help you in europe as it is the airspace that requires you to be FM immune not the state of registration, I cant see a reason that a "G" plate aircraft cant fly IFR in the USA without FM immune equipment as it meets all the requirments of the states airspace.