Hate to break it to you, but in Brave's case you really can't. Their crypto and ad stuff is so ingrained in their codebase that you have to be pretty meticulous to wipe it all out on that level. https://github.com/brave/brave-browser/issues/14783
But I was mostly interested in the theoretical situation if brave forked from the servo stuff and not its existing chromium stuff, no interest in touching that.