Please trust @fefrei (as well as all of us on the team who develop Discourse) that you do not want to fork the codebase. It is a recipe for disaster, especially in regards to security fixes which could become difficult for you to patch if they touch the code you modify to override the username rules.
It sounds like you want users to see the names (with spaces) that they’re used to. Why not prioritize full names (see site settings) and ensure that usernames are copied to that field, which allows spaces. Then SSO can manage the username field.