You really need to see a NY lawyer on this. Each state is different when dealing with an issue such as this.
If you haven't sued them already, you may want to do that depending upon how much money is involved. Most attorneys would not take a case like this on a contingency basis. And they would probably want a good size retainer to work against.
If you do sue and get a judgment, there are a number of ways to go about enforcing the judgement to get paid. But before I sued them, I would try to find out if they are even collectible. Because if they aren't, you are just wasting time and money.
One final option is a mechanic's lien against the vehicle so they can't sell it until you are paid. Again, each state is different on this so you need to consult a local lawyer.
Good luck!