Plywood, once bent, is loathe to return to planar.
This is handy in boat building, when plywood is used for planking.
Not so very helpful when used to create a skeleton upon which one wishes to create a symmetrical hull form.
Your best bet remains either getting a replacement piece, or using the existing one to make a template on suitably flat material.
even if you could get 90% of the twist/warp out, it would still be there in your mind. So, cutting your own replacement is a way of making that self doubt "go away." As would be getting a replacment bulkhead.