I tried a few different solutions for fitting spool pins to my 431G.
The best solution I found was to use double chainring bolts that were designed for bicycle sprockets.
These chainring bolts are 10mm in diameter and have a small 3 or 4 mm hexagonal hole in the top. They fit well in the 431G top.
The attached photo shows similar items on Amazon.
You may have to try a few spool pins to get the correct fit. I just made my own out of HDPE cutting board material on my hobby lathe.
You may have to cut or sand some of the finish off of the inside of the spool pin holes to get the chainring bolts to slide in.
You will need a small spacer to hold the top of the chainring bolts flush with the top of the machine. I bought a nylon bushing at the hardware store and cut slices to fit.