In your first sentence you mentioned 'street rod' . . . if that is the case, then you're probably not concerned about originality (like a lot of us would be). The next question is do you really want to run a flathead? Most of us on this site would encourage you too - some on the HAMB would not. It comes down to what are your goals, what skills do you have, what $$$ do you have to spend and are you into it to learn a lot along the way? Flatheads are really neat engines - NO, they're not like a crate motor SBC that you can pickup for $2000 and have an instant 350 HP and parts at every local store.
On this site, most all of us here would encourage you to run a flathead -- if this is your goal, then there are a LOT of ways to skin the cat. If it was me, my first work would be to pull the original engine, take it apart and see what you have. Until you know this, it is hard to recommend whether or not to rebuilt it (maybe with a few minor performance upgrades), or push you toward the last generation 49 - 53' engine with the more modern advancements that come along with it.
Take your time, get into what you already have, buy some books to learn, ask a lot of questions (we have some real experts here) - then we can all help you understand costs, performance options, work, part vendors, etc.. Good luck.