32mm square is too small a section to be attempting to pull this thing back into shape in one place. You'll just break the frame at the end of the brace.
You could try this:
-Screw a 4x2 across the outside bottom of one end of the shed such that it protrudes 6 inches or so either side of the building. Screw another one at the opposite end of the shed at the top. These will act as load spreaders. Then place a ratchet strap on both diagonals, and tighten in unison. Pull the building ever-so-slightly past true, because it will want to relax back towards its previous racked position, then fix it internally either with 4x1 diagonal bracing (think carefully about the door), or internal sheathing board (ply or OSB). Then release the ratchet straps.
BTW, if this does cause the roof to leak, replace the felt with EPDM and never worry about the roof again. It is a simple DIY job and cheap.