Greg Bender
http://www.thisoldtractor.com/gtbender
There is a tiny little spring loaded ball bearing used to make contact between the advance plate shaft and the main shaft that is connected to the gear. The size of this ball bearing is 3.175 mm or 0.125" or 1/8".
A suitable replacement is available from McMaster-Carr. Search for part number 9528K11.
If you need one, let me know as I've got plenty. greg>at<thisoldtractor.com