My 431G came with a messed up zigzag needle plate. Someone had drilled it to accommodate a flat-head screw.
Replacement needle plates for Singer 431G machines are extremely hard to find.
I successfully modified a needle plate from a Singer 700 series machine to fit my 431G.
I ordered a Singer 174198 needle plate from eBay. The ad said it fit models: 750, 756, 758, School 417, 611, 719, 775, and 776.
The plate is very similar to the 431G plate. The holes are in different locations and there is additional material on the back side of the plate. The feed dog slots are also just slightly smaller than the 431G plate.
431G plate (left), 174198 plate (right):
Here is a picture of the plates stacked together.
Here is the new plate with changes marked in black.
I drilled the new plate and sanded the extra material from the rear edge. I used a small diamond grinding bit to expand the feed dog slots and to make a few final adjustments.
The following pictures show the modified plate which fit into the 431G with the new magnetic mount. This is a hand-made prototype so it has a few imperfections but it worked very well. I intend to use this instead of the zigzag plate that came with my machine.