That's a very simple project that can be made with a flat washer, a few needle files, and maybe a cutoff wheel on a Foredom tool. I've made the entire latch assembly including the turn handle on several sporters over the past 40-somerhing years. Any competent metalsmith should be able to turn out the locking tab pretty quickly. Jim Carmichael describes the entire process of building a hinged floorplate from scratch with step-by-step pictures in his book "Do It Yourself Gunsmithing".
Jerry