The block is beautiful. Is this a single block as an accent block or will it be used in unison with others?
If it is a single block to be used as the accent block in a topper or other such thing – I would be tempted to leave it alone and add a white border around it to square it up.
If you are making more and need them to be alike, I might try to look at the seam that goes from top right to bottom left and see if you can adjust it there. If that is a continuous seam you may be able adjust up to a ¼ inch there.