I'm no expert but as far as I can see, as I said, there is only one size available. I have an M7 x 40 on my GP which is the right length. With all the different variations on remade kickstart pedals I presume that you would have to make do with what is available and trim to size if necessary. Perhaps one of the long time established dealers/engineers on here might know different.
The early Innocenti parts catalogues used to give a part no based on a)the location on the scooter and, b)the size of fixing but I understand that this was discontinued after the Series 2 style.
Quick measurement of SX kickstart with a broken bolt showed that the lenght of the bolt "inside" of the pedal was about 37 mm, so M7 x 50 mm might be a good quess, as you probably want to attach a nut and a washer to the other end of the bolt.