Maybe you can find the model you need in ADS vendor libraries, such as:
http://www.home.agilent.com/agilent/editor...&id=1490117If this doesn't suit you, you can use Log detectors from ADS "System - Amps & Mixers" toolbar and configure it with number from data sheet (slope, intercept...).
Alternative to this is to use Log amp Verilog-A model which models saturation and other effects and use the numbers from data sheet to make it work as close to the real thing as possible.
I doubt that you will find Spice models for these circuits - at best they you will find a behavioural equivalent - not the real netlist.
If you're interested in using ADS Log amp or Verilog-A model, maybe I can help you.