To be honest it doesn't make a difference if it is a NX motor or a G20 motor, it is a SR20. Chances are the shop or person would not get a high mileage motor and drop it in the car. Most salvage yards try to sell lower mileage motors. If it were a Soken motor it would be better. The only way I would see someone swapping out valve covers is if they were installing cams or something and dropped the valve cover and broke it and got one from someone or a salvage yard. I understand your confusion but in reality if the motor doesn't knock it really doesn't make a difference. Enjoy the NX2000.
Check out my page in the sig! Lots of ideas there.
As far as Carfax is concerned, I will be the first to say they are full of ****.
Yeah, it will show you when the car been inspected, but my car was written off by two insurance companies and it is not listed on carfax. My brother bought a 907A car (NY Salvage car code) and when I ran the VIN on Carfax it did not show up either. So DON"T TAKE CARFAX SERIOUS!!!!