I don’t believe it is correct to call Ayn Rand a conservative, she is closer to being a libertarian. Conservatism in America today is firmly attached to the view that America’s traditional Judeo-Christian values should be maintained and strengthened. It would be difficult for an atheist to hold that position. Of course, anyone, atheist, agnostic, or of any other belief (or lack of belief) should be welcomed if they choose to ally themselves with Conservatism in the struggle to save Western Civilization.
You make a good point on Ayn’s politics, I should have thought of that.