There are many ways to cacualte NF in Hspice.
In standford's webstie there is a detailed example.
You can find for it.
But as I try the equation it gives, I think there should be some modification.
However I'm not sure what is right cause they don't give netlist.
You could just use their way and edit the quation as your understanding of NF of LNA.
When stability is in concern, I think that depends whether you would make 50 Ω output of LNA. If you do, then you could just measure the S parameters and caculate K.
Of course my method needs much extra caculation. So if anyone could give a direct way, I would really appreciate!
Regards!