Eric,
The magneto gear has 32 teeth (so that it runs at half engine speed). The idler has 29 teeth as it only has to transmit the drive to the mag - it doesn't drive anything else that has to run in time with the crankshaft. Also, it is good engineering practice to use an idler with an 'odd' number of teeth, so that any wear is distributed around all the teeth rather than concentrating it on the teeth that are in mesh when the load is applied - not a problem with mag drives, but cam drives with their higher loading are a different matter.
Regards,
Eddie.