Well, firstly it depends on what era and client version you are aiming for. You'd have a better shot at building a "true" UO shard with older versions (client versions). Most of the repos floating around that I know of (RunUO/OrbUO/ForkUO/ServUO) are accurate up to Stygian Abyss. I know that not all SA content has been created, so that's probably where you would want to stop (right before Stygian Abyss).
After that, it's just a question of setting up your shard and the World Spawner takes care of most of that for you when you use it.