If you are refering to the chopper rotor itself, Rodono makes a nice one with flails. As for the original chopper rotor I don't really know of any aftermarket knives of a different design. There are alot of aftermarket sections available for chopper concave that are more aggressive and use knife sections. We make our own around here. Main thing for chopper hammers is to keep them in good shape. Once they get rounded, they won't work as good. I have tried building edges up with hard surface weld, and that works pretty good. I will be trying a set this year on a combine that were cryogenically frozen to see how that works. It has worked good on knife guards and threshing elements, so I don't see why it won't for chopper blades also.
We weld the case IH sickle sections to steel plate and bolt them in. Also modified our knife bracket to get it up in the slots as far as possible to get more knife exposed. Also drilled bigger holes for a 1/4'' bolt so they don't break off so easy. Works great in bean straw, as long as the rocks dont get them.