Thing about the Kon-Tiki is that Heyerdahl has been proven to be completely wrong by DNA evidence. There's not a scrap of DNA from South Americans in the South Pacific Islands; Polynesians are all from Taiwan and Southeast Asia.
That's fine and dandy. But the journey could be made, even using very primitive methods. Where they came from is a different story. But once they caught the trade winds or the ocean stream or whatever, a primitive ship could have made it to the Americas.