I am going to take a guess here, the part under the curved piece and down, in the picture, from the gear on the main shaft, looks to me as if it had a plastic or rubber gear on it which obviously is no longer there. I say this because the surface of that gear, with all the small points, is what I've seen inside of other rubber/plastic gears, the purpose being to keep the gear from slipping on the shaft.
In my non-expert opinion the likelihood of finding a replacement gear, if it's what I think, is so statistically insignificant that we can call it "zero".