That's a tough break, amd it really sucks that there are clowns like this "losing" (read: stealing gear) their customer's equipment and then ducking responsibility. Not the first such case, there was Noah Schumacher a few years back who scammed so much gear he actually got prosecuted. Some gear was recovered and returned to the rightful owners, but much wasn't, and many people were just out of luck. Before that there was QuadBob, much of the gear he failed to return was never resolved satisfactorily, to my knowledge. Further, no case was ever brought against him.
The bigger problem is that without the documented serial number (and legally satisfactory proof of your ownership) I don't think you have any chance of getting it back even if you do find it somewhere. How will you be able to convince police that a unit you spot is actually yours? You'd need something to establish ownership, and not just a statement "hey, that's mine. It has a distinct scratch in the lower left corner of the faceplate" (or whatever means you can use personally to recgonize it). Point being that no police dept will endavor to recover it for you without some way of provably establishing it was actually yours.
If the guy is local, I'd probably stake him out and confront him when he leaves the house. He has to go out sometime. Of course he'll avoid phone calls and other official attempts to make contact, he's a chickens***. I'm bold enough I wouldn't accept that, and would put that skid mark on the spot.
That might not get my stuff back, but I'm not above using a little intimidation on a crook even if just for the entertainment value of it. Yeah, I know that isn't a popular position, but nobody screws me and gets away clean with it, whether it gets resolved within the law or not. But enough said on that.