put mas file in rFactor 2\Installed\Vehicles\Porsche_911_GT3_R_2018\x.xx folder where x.xx is latest version's folder you have in there. when S397 updates the car just move the mas file in new version's folder
added compatibilty with new UI, i recommend to delete all files of v1.9 (folder and rcd file) and install the new rfcmp file by putting it in packages folder and installing from game.