Cameron, for the drive sprocket side bearing the R&O specifies Fafnir 107A, I simply purchased whatever Kent had, and I see Walker's and Greer's also carry one too.  Sorry I don't know the bearing number but I measured the new one compared to the old one and the new one was spot on.

I can only recommend that you take as many pictures of the gearbox and how that side is assembled before you tear into it.  I didn't do that and when I put mine back together I wound up having to reduce the spacer collar width by .02 because it wouldn't go back together with the proper clearances....and I didn't know if the clearances were on spec before I disassembled it!

As a side note, I never originally used my gearbox...my engine was toast when I got it so I don't know if the gearbox was good, or if it had been rebuilt with wrong parts...the only new parts I needed were bearings, bushings,  and the slider gear.