I purchased this one, waiting for it to arrive from China. The spacing on the mount is 140mm which should fit the existing holes for the grab handle. Hope it arrives soon, should be a fun little project.
https://www.ebay.com/itm/License-Plate-Bracket-For-Yamaha-MT-09-FZ-09-2014-2018-FJ09-Tracer-900-2015-2018/282839218700?ssPageName=STRK%3AMEBIDX%3AIT&var=582205486979&_trksid=p2057872.m2749.l2649