Can an overseas startup copy a US-patented startup?

The protection of IP is territorial, therefore if it has been filed and granted in a certain Country the IP title belongs to the Applicant(s) in that Country and may sue anyone in order to protect their rights there.
Where there is no IP title anyone may copy and commercialize it.

While you have not noted that the technology is or involves, for example, software or similar subject matter, it sounds like that is what is patented and covered by a US patent. That being said, note that a US patent prohibits others from making, USING, or selling the patented subject matter within the US. Therefore, if somehow there is "use" within the US, there may be infringement. Consult a patent professional experienced with such types of patents for further information and ramifications.