“Taharas HaKohanim Kehilchoso” 12 (2) (recently published by Moshe Gross, Beit Shemesh) mentions that there is a machlokes about the rabbinic issur of kohanim to be metamei themselves in “Eretz HoAmim”. He quotes a number of sources including many Acharonim that are lenient (including those quoted in the first answer).
He then quotes sources that forbid a kohen resident in Israel to leave the country. He quotes “stimas Tur Shulchan Oruch”, Shu”t “Shevus Yaakov” 1 (85) and 2(98) and says that Shu”t Chasam Sofer YD 337 D.H. “Vehinei Beshulchan Oruch” is inclined to be strict.
Those already living outside of Israel are already tomei and considered as anusim.