Unfortunately, the diameter of the machine gun barrel in 1/35th scale would most likely dictate scratch building a newMG42 around a fiber-optic strand.
Probably making a mold of the MG-42 and clamping a fiber optic strand in the center of the mold.
However, the lack of space between the fiber optic strand and mold interior would probably make injecting the casting material necessary.
If that can be accomplished, then the fiber optic can be lit with an ordinary flashing LED circuit hidden below the diorama.