The police may also be able to help you recover the car, since you still have the pink slip.
You will likely need to bring a lawsuit to collect what you are owed. You may be able to sue for return of the car, since you are the legal owner. In any event, you probably should write a formal demand letter to the person who owes you money, demanding payment in full or return of the car.
If you are truly worried about the insurance, it may be possible to purchase an insurance policy. You could then sue for the premiums for the period of time that you do not have the car.
I realize that none of these are great options, but I do not believe there is much else that you can legally do.
