Mark is correct.
The gearboxes must be metal since they are electrically "hot" and carry the electricity from the trucks to the motor-- either by jumper wire or by the frame itself.
Even though it is soft, pewter might be a way to go- as far as making castings and copies.
And just for reference sake, not all gearboxes are identical. I have seen three versions.
The slot for the gear-retaining headless screw is either level with the frame (E units, for example) or it drops down (ALCOs and SD40-2).