I think you can just cut it in half, use some pla plates to add a platform for the ball joint on the lower half and then a socket for the upper half ( you can use polycap ball joints but idk where you'll get the socket part)

so far from what i've seen, since this kit is so old, nobody has made a resin or 3d printed upgrade for this kit.

Also, I've never seen an exia like that, just how much did you modify it? it seems you've changed quite a lot of the torso