Darkling Dragon has been running UDICraft, an Ultima-themed Minecraft server, since about 2013, and has slowly been refining and adding to it as time permits. The server offers, more or less, a complete re-creation of Sosaria (although some areas of it are still being worked on) based on the heightmap of Britannia from Ultima Online.

You can find what I think are the essential bits of additional content that are required to play on the UDICraft server at the newly-created UDICraft project entry (the server address is also given thereat). A more comprehensive list of downloads can be found on Darkling’s website, although I suspect that some of the files listed there supersede other, earlier files.