The police are not a debt collecting agency, so they wont get the money back.
They will deal the person for theft and deception, but will leave the penalty to the courts.
Its not clear if 'the other guy' the new keeper or the original seller?
Has the car been recovered yet?
If the other guy is the new keeper, and he has been out and about in it they may not have had chance to see him. Ok, it will flag up on ANPR (if he goes anywhere near one), that still relies on the police being somewhere near when he does in order to stop him.
If the other guy is the original seller then there is no rush as such to go and see him. The officer in the case (OIC) will work shifts, so may have been going off work at the time of the report and may even be on their 4 days off now. Its a long process, so give it chance.
Another thought could be that the original seller may have also bought it off someone, so could also be a victim for all you know.
Yet another scenario to consider, how did he pinch it with keys AND document? A possibility could be that he paid by cheque that has bounced. He may not even be aware of this fact, so may still be an honest mistake.
Give the investigation chance to run its course.