Lost only key?

Nissan antitheft systems can be a bear to deal with if you don't have a factory level scan tool to deal with them. I've fought with them a few times even having the correct tool. In this case the easiest method would be to go to the dealer with your ID, your title and registration for the car and they should be able to get you a key that will fit your car, then they can program that one in. From there you can have extras made and the lost originals will be erased so if they do turn up, they will be useless.
The idea of installing a new cylinder is valid but you would still need to have the old keys erased and the ones that came with the new cylinder programmed in. Plus, without a working key, cylinder removal would likely damage the housing and cost you even more money. The replacement keys also won't fit the other locks on the car so you would need to pull those and re-pin them to match the new keys. I wish they didn't make it difficult, but it does seem to keep thefts lower.