I have an XX1 cassette, and broke the cog with 24 teeth, is this replaceable or repairable?
So you broke the 24T cog in the middle of the cassette? If that is the case i don't think it will be repairable. You may check with Sram Warranty though dependent on circumstance. I have had good luck with their warranty program in the past.
My reasoning for saying that it is not repairable is that on the XX1 cassettes, the cluster is machined from one solid piece, except for the largest cog which is made from alloy rather than steel So they are not all independent cogs, but rather one large intricately machined piece, making single cog replacement impossible unless it is the largest cog.
From Bike Radar:
Many of the lessons learned from developing the Red and XX cassettes were applied to XX1. 10 of the 11 cogs are machined from a single block of tool steel. Manufacturing begins with a bell-shaped forged blank. This blank is placed onto a lathe that removes a significant chunk of material and leaves the blank with a stair step profile. Next, a CNC machine removes more material by carving the teeth, shift ramps and the 16 pins that are used to secure the 42-tooth aluminum cog to the steel cassette body.
From Pink Bike