I don't think the other two panels are available, unless you're lucky enough to find someone selling new old stock.
It's time consuming, but not too difficult to repair though, because only the top panel is visible once repaired.
I repaired this area on Ste Haigh's 240. Instead of recreating the rust-prone triple layer seam, I joined the panels by seam welding and made repair sections to fit accordingly. The end product looks original where visible, but where it's out of sight it will resist corrosion much better and longer than the original spit welded joint.
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