replaced a engine with a used engine

I have taken my 2004 Hyundai Sontana to Gandara's Auto Repair several times before. Back in the last week or so in January 2012, my timing belt broke (which also my check engine light was on). Had the car towed to Ganadaras. I agreed to have the work done which included a used engine with 68000 miles. They replaced all the belts. It seemed like it took them forever to fix it. I was also told that the clutch needed to be replaced, I havent had it replaced since I bought the car used a few years ago. Since picking up the car early this week Feb 2012 I have had to take my car back twice. The first time was because the check engine light came on. Second time was because the gas pedal would stick for a few mins. Now I will be taking the car back this week, the car is leaking small amounts of oil and the check engine light is on for the second time since they fixed. Hopefully after taking it back this week there wont be anymore probelms. If I have anymore probelms with my car I wont be taking the car back there. Should of taken the car to the dealership.