PROBLEM SOLVED!
Thanks for the link, Tiger. But unfortunately I did not find a replacement hatch cover.
My solution was a Vetus ILT universal inspection port which, after removal of the old flange, perfectly fits into the old hole. The Vetus ILT is according to my opinion much more reliable than the white inspection ports you can get everywhere and doesn‘t require :copdrilling of additional screw holes because it is fixed by compression of a rubber gasket.
Perhaps helpful for someone with the same problem.