They can surely do Teshuva, at least enough to spare themselves from punishment on this world. This is seen from Ninvei and the fact that Paroh’s choice for Teshuva was taken taken away.
However, it says in Pirkei D’rebbi Eliezer (IIRC) that after somee time Ninvei went back to their bad ways, since they aren’t really able to repent.
As to Olam Haba, it says, Kol Yisroel Yesh Lahem, not Kol Haumos. Afterlife is not Olam Haba. They do have an afterlife. If they do Mitzvos L’sheim Shamayim they get Olam Haba, too. This is what I’ve heard B’shem the Rambam.