I absolutely agree that GTAW (TIG) is the best process for this, however, if you MUST choose between what you have, use the wire feeder with .023" wire and use C25 as a shielding gas if you can. 100% CO2 may also work but not as well on small parts. You didn't say just how small the part was, which is what is really going to make all the difference. I used this exact procedure to repair armature shafts on Ford Model A horn motors when I didn't have access to a TIG at the time.
Practice on something very similar first, if you can. Don't hold the trigger and expect to let off when you think you've added enough. Hit-and-release the trigger fairly quickly (make sure you've already purged the gas!) and add just a little metal at a time.
The strength of the needed repair is going to depend greatly on what part you are repairing, of course. We are assuming it doesn't need to be heat-treated.