Hi Jason,
I'm afraid that only the magnitude part (dB) of RF and LO s-parameters are available when ADI characterized the part. They're published in figure 27 and 28 of ADL5367 data sheet. In your sims, you may want to treat the mixer inputs having good 50 ohms impedance.
ADI does provide IF output impedance in form of R//C model, so you can set up your filtering between mixer outputs and ADC inputs when desired.
Regards,
Rui