WASHINGTON (Reuters) - The Pentagon may want to consider scaling back Lockheed Martin Corp's multinational F-35 fighter program, the costliest-ever U.S. arms-purchase plan, as part of stepped-up budget belt-tightening, an analysis by an influential research group said. The private Center for Strategic and Budgetary Assessments, several of whose one-time experts are now serving in senior Obama administration jobs, cited the F-35 as just one example of programs ripe for review by the Department of Defense during its once-every-four-year, top-to-bottom re-assessment now under way.