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.