I'm not sure if I understood ya correctly, but in general, modifying the MTL in the MTkit won't work as it's not compatible with Crash Time 4+.
Your only chance for editing the MTL is by using your favorite text editor or you'll use the MTLWriter by thecloser_ (which ain't available for download right now, needs to be fixed ^^).
In case your livery ain't showin: There is a flag in the MTL file for textures. You have to choose one there. I think for Crash Time 4, mostly "02 Transparency Specular" is used by the most people (see example below).
As I'm not an expert in this kinda stuff, someone else should answer/correct me here or give better instructions ^^
Example MTL code
MatClass 00 02 00 04
Diffuse 0x8a8f96
Ambient 0x000000
Specular 0xb8bfc8
Reflect2 0x969696
Specular2 0xb8bfc8
XDiffuse 0xffffff
XSpecular 0x000000
Tex1Name "livery.tga"
Tex2Name ""
Tex3Name ""
TexFlags 0x11 0x00 0x00 0x00
TexOffset 0.000000 0.000000
TexScale 1.000000 1.000000
TexAngle 0.000000
Alpha 000
SpecProps 200 010 059
Fresnel 095 025 080
FallOff 000 010
🤙