I'm building a Round deck currently. Some of the cards are out of format, but I found how to replace them.
But, what can I replace with N? (because it's out of rotation)
If you wanted to future proof your deck - and even then Round is going to be moved out pretty soon - you could run Prof. Birch's observations in place of N; I remember seeing a forum debating it on here, so you may be interested to look into that thread.