2003 Saturn Ion engine cooling fan

Recently I had the engine replaced after 112000 because one of the head gasket cracked. I ended up getting a different engine with fewer miles instead. The Fan Does not seem to be working now and the car over heats when the car sits for a good 15 minutes or if i'm driving at 30 mph or less and have the air conditionning on. I can hear that the thermostat kicks in for a bit but then it shuts off and the fan does not turn on like it used to before. Should the thermostat and coolant sensor be changed considering it's a different engine? Where can I begin looking to find out what's the problem?