HONOLULU — Hawaii would be allowed to opt out of key requirements of national health care reform legislation, the only state given such a privilege because it already has its own comprehensive health insurance law. Hawaii’s exemption is written into three paragraphs of the 1,900-page House bill that’s set for a vote Saturday. U.S. Rep. Mazie Hirono, D-Hawaii, said Thursday that Hawaii’s 35-year-old health law should be preserved because its requirement that employers pay for full-time workers’ health insurance has boosted the number of insured residents to one of the highest rates in the nation. ‘‘We have this exemption because our prepaid health care law is better than whatever the federal government provides,’’ said Hirono, who introduced the Hawaii amendment to the bill. ‘‘Hawaii has a very progressive law on its books, which no other state has tried.’’ A similar employer requirement in the federal legislation would make workers pay more, Hirono said.