I recently fitted facelift Sports seats in an 03 car. With
@Usel's help I fitted a seat pad emulator, swapping the occupancy sensor over from my pre-facelift seat and then attaching an emulating resistor to it instead of plugging it into the seat. This makes the car think there is someone in the seat all the time so it wants the seatbelt clipped in, or else it bongs at you.
You can either attach the seatbelt, get a spare seatbelt clip and fit that, or use NCS Expert to tell the SBSL not to worry about the seatbelt clip.