distributor question

[vk2icj]

Ok I was just looking at my distributor to get an idea about installing my Pertronix kit. I noticed that the cover under the rotor (not the cap) has a big crack that almost runs the whole diameter of the cover. Could this be causing my rough running? Can I buy a replacement at REPCO? What about Nissan or do I have to get a whole new distributor. I've only seen caps for sale in the shops.

Chris