As stated CIVIL MATTER, unless you can show he had no "intention" of paying when he asked for the repairs, then this would be "obtaining goods or services by deceit", which is a criminal matter.
You are 100% legally within your rights to take the car so long as you cause no damage, I have done it many times,(repo work) police have been involved many times, but so long as you can SHOW a debt is owed by cars owner, you can keep it for 14 days subject to his paying upon which time if he has not you can sell the car to recover your debt, you have to give him a letter to this effect though just to cover yourself.
Most of the cars I repoed had no court order.
tabetha