They didn't so much racebend Namor, as change the Atlanteans to water breathing Mayans. Namor isn't white. He was half white. My initial thought was that Namor was still going to be half Atlantean, but I'm not so sure now. He could be half Mayan and water breathing Mayan, which could still make Namor bi-racial. Or he could be half Spanish and half Mayan, which would also work. Or, as I'm beginning to suspect, he's actually going to be fully water breathing Mayan, and won't be bi-racial at all. His powers will all be due to mutation. Needless to say, I hope they don't go that route.
Kukulkan has no history with Black Panther. Nor is he human, subject to all the trauma that they want to put Namor through. I think, he'll be far more like Namor, but as Imp pointed out, his Atlantean half will be Mayan, instead of a fantasy minority.
Yeah, I can honestly say I've never met a Kukulkan fan. I think he's only appeared once, maybe twice, as a background character. And MCU Namor is not taking his identity. He's not a god. His people just treat him like one, sort of like Storm when she was in Kenya.