02 Nissan Altima 2.5s

I just had a head gasket job done and when they replaced everything they didn't time it right so my car does a little bit of a jump fells like its goin to stall out at red lights but doesnt, rough idle, along w. That I'm having white smoke out my exhaust. Smoke only comes after car is at runnin temp and gas pedal is pressed. Not clouds but its not normal I was told have the timing chain timed correct and the oil burrning issue is most likely valve seals or valve guides? Can someone please give me a 2nd opinion on this? And if the valve seals or guides is this a hard job to get done? And also is retiming the timing chain a hard job? Thanks in advance.