The only source for vintage Craftsman 109 lathe parts I've found is www.homeshopsupply.com. He has the gears, but they're very expensive. I'm sure that the site owner would be more than willing to give you the dimensions of all the gears. You can also try the Yahoo group at http://groups.yahoo.com/group/AA_109...guid=192410164. There is a lot of info about these lathes at both sites.
I have this same lathe that's also missing it's change gears. I don't thread with my lathes so I really don't miss them. I'm in the middle of a major rebuild/update/modification to make it more precision and easier to use than stock.