For FS9 there's pretty much nothing to do, the CFS2 models will work quite well. In fact, the default CFS2 planes were part of my FS9 hangar (Corsair, Hellcat, Wildcat, Zero etc...)
For FSX however, it's more complex. Some CFS2 models, depending on which tool they were created with, are incompatible with FSX, which won't be able to load the model at all. It's not a texture problem, it's really with the MDL file format. And there's nothing that can be done about this, excepted if you have the source model and convert it to a FSX-compatible format.