Kato sells the shells, but they're down to BNSF versions only.
KCS units have high-mounted numberboards.
FXE units have low-mounted numberboards.
The numberboard placement is a fairly distinctive difference between the two and Kato made models for the both (but both are sold-out at Kato). But, in my opinion, the gull-wing cab difference is a bit less noticeable. In my opinion you could get by purchasing a Kato Undec Gull-Wing model and painting it for KCS. Relatively few would notice what you did and even fewer would outside of a full-frontal shot of the model (gull-wing being most noticeable from straight-on). There would undoubtedly be a lot more errors on the models for the nit-picky outside of you using a BNSF cab.
If you don't know what the "Gull-Wing" is, look at the below picture. Notice how the roof-line on this unit has two seperate angles on the ends. All other units have a single ~45* slope at the ends, but the BNSF cabs have a gull-wing notch two-angled slope on the cab roof edges.
