I have a spare key, can I repossess the car myself.

I've sold a car for 1000.00 last November 2017 on an agreement of 200 a month until paid off. I've only received 350 of the 1000 and the last bit of money I got out of her was at the end of December. Almost 5 months now, but what's concerning is I signed the title over so that new tags could be purchased under her name. She's telling me the car needs a new transmission and rear bushings and all sorts of small stuff, but still takes it to work everyday so I know it works and has worked since I handed over the keys. I found a spare key to it and know where it sits, can I repossess the car myself or do I need to go to small claims court?