spice model from equivalent circuit

I have the equivalent circuit of the HEMT device as part of the datasheet. How can I create a SPICE model for the transistor using the given equivalent circuit model?

If I am to create a circuit in ADS with the attached equivalent circuit (given the value of all elements including Gm and tau), how should I specify the current source in the circuit?

First, which model you would like to use or in other words, which model fits your requirements ? Because there are few model such as Curtice,Angelov,TOM,Skallern-Parker etc that characterize HEMT transistors and every model has advantages and of course disadvantages.If the model parameters are already given, you may use Generic nonlinear HEMT model in ADS.I don't recommend to use other models because since you don't know the other parameters, the simulation result will not be correct.
For your circuit ADS_FET_Model seems good.For more information, look help file. ( To able to define other nonlinear components, you should use Data Access block).