The OP can prove that he has the private keys to the account the money was stolen from.
That means that we have two people claiming property, which is way better than nothing.
So, it ends in a Mad Max style chainsaw fight?
That doesn't change much for the person who got ripped off, or possibly for the person who he is trying to rip off. Proof of original ownership doesn't preclude a willing transfer going sour, greed after the fact, or some kind of elaborate collusion scheme (target address owned by friendly) for example hoping to solicit sympathy coins, or if you're paranoid even as a recon exercise into the exchange and/or pools.
Also, what happens if Mt. Gox complies easily, freezes target accounts? Do they have the right/obligation to do so? If so, scammers will try to abuse *that*.
I'm sorry for your loss, but am dumbfounded by the nonchalance with which that kind of 'f*ck you' money is treated. For lots of people a sum that large would mean never having to work again (unless the fed manages to lower interest rates to zero of course).
I'm also very skeptical about 'following the money' (blockchain), every time there is a split or merge all in/outputs are 'infected', at some point making it impossible to follow, let alone prosecute to any meaningful conclusion (do you average your loss over every recipient?).
The only recourse would be to physically trace the perpetrator and get them to admit the theft. Which on the downside would put a dent into the pseudonymous image Bitcoin has.