Mods to my Big One are listed here, to give you an idea of things you can do. Not sure how the pricing would come out compared to a new Nimbus Impulse Shadow, probably still less, but no disk brake and heavier for sure.
Also worth noting that I don’t think the Coker frame will fit the Nimbus hubs, so if you’re really looking to a disk brake, this would not be the way to go.