https://www.beamng.com/threads/mods-section-rules-updated-july-2020.61804/ Normally, this would be a case of "Assets from other users [1]" but, if you read [1]: "[1] The user being part of the BeamNG community or others, as far the user is active / reachable." as the original creator is neither active nor reachable, this would be a case of " Assets from other sources with credits [2] Uploader takes all responsibility for what is being uploaded. Thread will be taken down if original copyright holder performs a copyright claim " [2] says this: "[2] Credits are required whether the asset has been or not modified in any form or extents." Still then, I'd ask the Forum Devs if its okay. You could also make the PBR textures a seperate Mod, and (maybe) give a link to the original mod.
you could just post the main.materials.json or materials.cs so you still have to install the main mod
or make it a "mod of a mod", so that it just overwrites the "original" files. just name the changed files the same as the files the should overwrite (they won't actually overwrite, but the game will load them instead of the old ones)
ey dude i doubt the author is gonna reply anytime soon, just post it. if he has a problem with it he will come back but his account has been inactive for ages so yeah i doubt he will really care