Depends on your mechanical ability, how much engine work have you done? If you can clearly remove the oil pan it shouldn't be to difficult. I would not pull the engine if you can avoid it. If you want to tackle it and your engine mileage is fairly low; by Nissan Standards less than 200K. I would get a Haynes or Chilton Manuel at the library if possible and check the procedures. You might be able to remove the lower cross-member and use a transmission jack to keep the engine in place while you drop the pan and remove the oil pump.
If you have a high mileage engine, you might be better off buying a crate motor. Japanese low-mileage crate motors are fairly cheap if you pay the freight from Japan. Try an internet to see what I mean.