IQUITOS, Peru — If Ronald Reátegui Levy someday finds that he is the last Jew of Iquitos, it may well be of his own doing. His dream, which he has vigorously pursued, is to persuade the descendants of Sephardic merchants who settled in this remote corner of the Amazon basin more than a century ago to reaffirm their ties to Judaism and emigrate to Israel. “It is getting very lonely here,” said Mr. Reátegui Levy, 52, an inspector at Peru’s national oil company, referring to the more than 400 descendants of Jewish pioneers who have formally converted to Judaism this...