You DON'T actually have to have a genuine N1, just enter it as such. But, you need to have all genuine N1 bits on there, such as factory oil cooler, N1 spec turbos etc. I reckon it'd be worth the exercise just to have that little bit of extra reliability up your sleeve.
Keep in mind that technically you cannot run a GTR with so much as an oil cooler if it wasn't on their from factory. Hence nearly everyone runs as N1 spec.
Btw, great to have another Skyline there