Spare parts for IKEA Ektorp 2-seat sofa bed wanted

Hello everyone, my Ektorp lying surface has not stayed flat over time but has become slightly slanted/sunken at the top (head) and bottom (feet). Nothing is broken or damaged. I suspect it is due to the old plastic (wooden slats) or the straps. Ikea no longer offers a repair service for this product and I am desperate... Any ideas from you?
(I live in Munich). Many thanks!!

Hello,
there will probably never be any spare parts for it. A few good pictures could help to develop ideas for solving the problem.