It's also possible to define the frequency response of a SAW filter by writing s-parameter by hand at some critical frequencies.( as much as possible) then by using model composer of ADS, you may obtain a discrete lumped element equivalent of the filter.
Of course SAW filter will show a bandpass filter behaviour and to obtain a very accurate model is very tough.But it's worthy..
Normally, an industrial SAW filter has s-parameters,you can use them in model development..