Insert a new empty top layer. Pick from a mid-gray area the sepia tone from the photo layer and fill the top layer with it. Let the blending mode of the top layer be = Color
That alone can be enough because the well saturated stain becomes nearly grey (=sepia) and keeps its brightness:
I guess you want to delete the edge ares of the sepia tone top layer if the white edges of the scanned photo are not full white. As well you can apply curves to the edge area to make it white in the photo layer, only make a selection. Here the edge area is selected in the photo and turned to white with curves. ===> sepia doesn't affect the edges.
If the stain isn't bright enough it can be seen as grey. You cal fix it by making the photo layer to B&W with Mono Mixer, Boost red and reduce blue channel. It lifts the brightness of the stain but doesn't affect how the sepia in the top layer works.:
This is the result with no effect dialogs:
OOPS the image has one unwanted step too. The smoke cloud or a camera light leak in the right is thinned by increasing contrast locally. It's a layer duplicate in the bottom which has got contrast boost with curves.
The top layer is wiped a little off with the Eraser. The brush setting of the eraser is made smooth and low opacity. As well one could paint black to a fully white layer mask to make the higher contrast bottom layer gradually visible.