They are both corrrct terms, "implementer" is now the more common between the two Ngram. It appears there is no difference in usage between AmE and BrE where implementer is equally common Ngram.

Is there a word implementor?

Implementor meaning

Alternative spelling of implementer. The definition of an implementor is something that is needed or used to do something.

What implementer means?

Meaning of implementer in English

implementer. noun [ C ] /ˈɪmplɪmentər/ us. someone whose job is to put a plan or system into action: She worked at IBM as the implementer of a major software marketing program.