What approaches did you try already?
I think it would be much easier to make textures in external software like substance designer, quixel mixer etc.
If nothing except max is available, I would try to start by soft-selecting "front" vertices of dense mesh and applying noise with tiny value on X, Y and more on Z. Z here is frontal surface direction/normal.
Then displace it by some stone/noise/dirt/grunge texture from the web, maybe in few iterations using different textures.
Assuming combining a bunch of maps baked from resulting mesh in photoshop for diffuse is also not an option, I would try to make some sort of composite material with those maps as masks to blend several noisy patterns.
Would be a good reason to finally learn how to use DataChannel. Seems like it might help in this case, but atm it looks scary and alien to me =(