I was in the same boat as you once. It was a buddy's shop and he always gave me great deals on labor but he would fit me in when he could. Right after I bought my 73 I took it to him to figure some wiring issues out, the car ended up not running anymore while it was there and it didn't seem like he was working on it at all. So my new car was sitting at this shop for nearly a year collecting dust. I finally got tired of it and had him tow the car back to my house. Turned out to be a bad ignition coil resistor.
I know exactly what your feeling right now trust me! It's hard for a shop to work on someone elses project when they can turn and burn a bunch of easy oil changes or other misc through the day and make their money. I wouldn't bother with the cops or lawyer unless you really think the car is being stolen and have a bit of proof. My opinion is go to the shop in person, ask to see the car and whats going on with it, and explain to him it's taking too long and would like an actual date of when it can be finished otherwise you would like to have the car towed back to your house to complete it yourself.