Spice model, can't figure it out
It took some digging, but I found some definitions:

\$ \mu_n \$ is the electron mobility, which is \$ 1360 \frac{cm^2}{Vs} \$

\$ C_{ox} \$ is the capacitance per unit area of the oxide layer, which is \$ \dfrac {\epsilon_{ox}}{t_{ox}} \$

\$ {\epsilon_{ox}} = 3.45 \times 10^{-11} \frac{F}{m} \$

\$ t_{ox} \$ is the thickness of the oxide layer.

The only unknown appears to be the thickness of the oxide layer, which is already in your model (TOX = 9.5E-9), so you should be good to go.