Looks like those tubes that drop down below the engine are open at the back ends. I don't think I would worry about trying to get a brush inside, but if you're worried about internal rust you could maybe thin down some rust encapsulating paint a bit, pour a cup in the tube and turn it every which way to coat the inside and then dump out the extra. That would be after pounding the cracks together and welding them up. The pipes that are sealed both ends and no cracks should be fine, right?

Great find for $25, BTW!