You have a 99 ECU, which is a 16 bit ecu, but it was never chipped. The last ECU that you can get chipped is a 1997 200sx/Sentra ECU. 98+ is a lost cause.
You cannot retune a stock ECU, you can run a piggy back that would fight with the stock ECU. But that's mainly only good for fuel tuning and not for timing.
Now you are still on the old school OBD2 harness, all you need to do is fine a chipped OBD2 ecu and modify your harness (move some pins), and unplug some sensors. JWT has instructions on how to accomplish that.
32 Bit ecu's, along with new OBD2 harness only happened on 2000+ Sentra's and P11's.