A small producer from Italy, Ari has produced a cog that will replace the large aluminum cog on XX1 and X01 cassettes. This is the only cog that is made of aluminum on these cassettes, so in theory could be prone to wear out much faster than the rest of the hardened steel cogs. SRAM does not sell this cog as a replacement, so Ari claims to be the only one in the world offering these right now.
More interesting though is that the replacement cog is also available in a 44t size. The stock XX1 cassette is a 10-42 spread, so in addition to a simple replacement in 42t size, they also offer a 44t for those people that really need to get up that steep hill. The larger replacement weighs 94 grams, or 12 grams more than a stock SRAM cog, and the replacement 42t from Ari weighs 82 grams, the exact same as the SRAM. Made from Ergal 7075 T6, they are available now for €75 plus express shipping to the USA.