No screws have been stripped. Two were simply missing and so were replaced with correct thread screws. All screws in the photo are the same size, they just have different size heads.
By looking at your internal picture of the forend, the cocking latch at the rear end appears to be broken off. I think you need a complete new forend iron. You can deal with the screw issue by shimming under the new forend iron if necessary. Gun will not #### with that forend iron as it needs that piece to push the cocking rods back in the receiver when the gun is opened.