John,
If you have the magneto drive in the timing chest and the magneto itself, you should be able to work out the dimensions of the drive dog (along with the photo to work from). As you can see from the rough finish around the centre of the dog, it was never machined to be a neat fit in the centre of the drive gear. Just the thickness of the dog needs to be a reasonably neat fit in the slot in the drive gear. The round button shown in the photo is just that - a slightly domed sprung button that is retained by careful peening of the hole. This button just eliminates any backlash in the drive, as the magneto is actually driven by the other side of the dog. When fitting the magneto, you will find that it should sit on 4 headed pins that go into the 3/16" holes in the top of the crankcase - this was done in an attempt to reduce the heat transfer from the engine to the magneto.
Regards,
Eddie.