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There is no restriction on the importation of these dolls, although there could be a ban on certain materials (such as lead) if present in them. That's true across a wide range of products, not just sex dolls.
As for pornography, simulated child porn is not illegal. The Supreme Court, in Ashcroft v. Free Speech Coalition, 535 U.S. 234 (2002), struck down two overbroad provisions of the Child Pornography Prevention Act of 1996 because they abridged "the freedom to engage in a substantial amount of lawful speech." This was where simulated child porn was made illegal, but that law was struck down. So pictures of a sex doll, though appearing to be underage, would not violate any law.
As for manufacturing replacement parts, no, no one would need the manufacturers permission. Third party parts are permissible, generally speaking, so long as you don't use the name of the manufacturer to imply any connection, affiliation, approval, etc... (which would violate trademark laws), and so long as there is no patent infringement. That second part is key. If there's no patent protecting this, then there would be no issue, but if there is a patent that would be violated, then you could potentially be sued for patent infringement.
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