I think the easiest way to proceed is to start from some example already available in ADS; a good one to start from is MixerDiffMode.
From the ReadMe:
This example shows how to simulate a differential-mode Gilbert cell mixer.
Many RFIC circuits have differential-mode inputs and outputs, so this
example should be of interest to RFIC designers. Small-signal mixer
mode is also used here, which is a technique that allows the conversion
gain to be calculated quickly, when the RF signal is assumed to be
small enough not to drive the mixer into compression. A noise figure
simulation is also included.
Just open the example and replace the existing circuit with the one of your project, using proper model for the MOS (a similar one is already available for the BJTs).
Then you can use the same bench modifying the setting of HB, adding parameters and so on.
With a simple add you can do S-parameter also, sweep the LO power.
I hope it can help.
Mazz