So you have the two-stage clutch. We have seen a number of clutch failures over the years, but it was always on the drive plate, not the PTO plate. I would first check the freeplay on the clutch, it should have about 7/8" inch of freeplay before you feel the throw-out bearing start engaging the clutch fingers. Some people dislike the feeling of a two-stage clutch and adjust it to engage the travel lower in the clutch ROM, and that messes with the PTO engagement. So if you did that, start by going back to the original settings.
The over-running clutch could be the problem. To my knowledge, the Branson OR clutch is pretty robust. We have only seen issues when using a fairly big tiller in hard soil. As a Branson/Century does not have a true crawl range (on this model), the ground speed is a little fast for a tiller in hard soil.
Unfortunatley the Branson shop manuals are not all that great. They are helpful for the guy that is already real handy with tractor repair, but they do not give a step by step disassembly procudre for each componet.
Get ahold of Frank at Auto Accessories of Clovis. He is close to you and has sold quite a few Branson tractors. I know it is a little odd that he is primarily an Auto Accessories business, but he is handy with a wrench. He should have manuals and access to parts. I think if it is the OR clutch, the tractor will need to be split.