Hi Thomas,
I'm afraid that you would need the Product Key to activate FSX - there's no other way around it, other than continue to use FSX for the thirty minutes it allows you. Many years ago, when FSX was still supported by Microsoft, you could give them the number inscribed at the Disk centre & they would send you another Key code. But that won't happen now since FSX has been shelved by them. You will need to get another Sim. My suggestion is to purchase FSX Steam Edition, as that it cheaper than the others and it is FSX Deluxe with the Acceleration goodies. Or else, the better Sims are Prepar3d and X-Plane.
https://store.steampowered.com/app/314160/Microsoft_Flight_Simulator_X_Steam_Edition/
Chris