By David Shepardson SPARTANBURG, S.C (Reuters) – Boeing Co won support Wednesday from a Republican senator and a major customer in its bid to convince the U.S. Congress to extend the deadline to win certification of two new 737 MAX variants. The U.S. planemaker faces a late December deadline for the Federal Aviation Administration (FAA) to certify the MAX 7 and MAX 10. After that date, all planes must have modern cockpit alerting systems to be certified by the FAA, which would mean significant delays for the new MAX aircrafts’ deployment, unless Congress grants a waiver to extend the deadline….