Firstly what a bloody shame, but I think you're in for a lot of grief with this one? You say you left the vehicle for painting, was this with a company or an individual offering a service on the side? If it were a company, you could claim against them and it could be a claim on their insurance, however an individual working on the side is unlikely to have any cover in place.
However, for the Police to do anything, they would need proof, holding keys isn't proof, as the chap in question could have simply forgotten to lock it, and although this could then allow some of the damage, through easy access, I doubt it would be enough to get a prosecution, without which how can you hope to make this chap reimburse you?
I think it might be better to walk away, as I can't see how anything you do now can make things better, unless you feel the vehicle was clearly in this other person's care, and therefore you could I suppose hold him liable for allowing the damage rather than blaming him for actually carrying it out, if he has failed to keep your vehicle safe by locking it etc? If you took this course it would be a civil matter, and would be for you to sue him for the damage.
It's really annoying, as you most likely know what happened, but proving it will be a different thing, unless you have proof, i.e. someone who saw the vehicle being stripped, saw the parts that were yours on another vehicle etc?
From my own experience: years ago I had a Ford Escort. It was parked in my yard, shared with another business. One weekend, we went away and I left it at work on the Friday night. We returned Sunday night to find my car on blocks, and the entire N/S suspension and wheel assembly missing. We noticed that there was a Escort Van parked outside the neighbouring unit, with evidencer of the recent fitment of a N/S wheel assembly and suspecsion strut etc. A blind man could see it had been nicked from my car, but nothing could be done, I even had the Police dust it for prints etc. I know that the thieving scum who occupied the unit next to mine not only nicked the suspension strut off my car for a customer's van, leaving mine immobile, and even to the copper it was obvious, but there was supposedly nothing that could be done. ( so I nicked it back )