Not difficult, but tedious.
You'd want appearance.2da from the top hak (which also identifies all the models for creatures CEP has added) plus other creature-related 2da files, such as appearancesndset, creaturesize, footstepsounds, soundset, and tailmodel.
As a rule-of-thumb, the models follow the naming convention c_xxxxx, and are in core haks 2 and 6, with corresponding textures in core haks 1 and 6.
However, there are many exceptions. The hp_xxxx models which in core hak 3, the rest you'd have to hunt for. You'll probably end up with missing textures, too, which can be debugged by decompiling the models (if compiled) then searching the ascii files for the BITMAP keyword... which is, well, tedious.
This is assuming you can live without the the new races like wemics, brownies and skeletons and the many updates coming shortly in 2.6.