Hi like rear gear cassettes, is the chain ring on the zwift one cog replaceable?
Zwift doesn’t sell the cog by itself but there are many third party options for single-speed Hyperglide cogs you could use. Some assortment of spacers may be required to match the width but those are widely available as well. The cog is just a Hyperglide cassette with a single sprocket so there is nothing unusual about that in the single speed world.