Street preaching is essentially evangelization, not homiletics, so conviction might count for more than formal training. I preached a number of sermons as a lay minister in an Anglican setting, but that was quite formal, for an audience that had a good foundation in Scripture and theology.
You might look for a chapter of St. Paul Street? I accompanied them a couple of times. I found the group I worked with tended to reach lapsed or non-attending Catholics…or fundamentalist Protestants who wanted to argue, though.