Copy the texture.cfg from the CS_B767-300CF-II_CF6-80C2\texture.CS.2021 folder, and replace the texture.cfg in the DHL's texture folder, and I think it will fix the problem

The engine textures between the two models (GE vs PW) are different...and are labeled different (hence the black texture you're seeing).

Copying the default CS.2021 GE textures into your DHL folder will only show the default three-tone CS GE engines.

If you have a paint program, you could (fairly easily) create two new engine textures by just changing the colors using the paintkit and using the freeware DXTbitmap program.

That would take about 5 minutes.

Conversely, just find someone who has already done a basic grey shade repaint  (no markings) GE engine texture for their paint and copy those two GE engine files into your DHL folder.

For example, I have a Cargojet repaint (a slightly darker grey) posted in another library.