How difficult is this rust to clean up? Will I need a full replacement?
Depending on the condition of the pipe, which I can't assess from my house, you'll need to either recoat it or replace it.
Hit it with a wire brush and see what's left when it's cleaned up. If it's still solid, consider recoating it with galvanizing paint.
If it's junk, take a look in your attic for the nearest joint. Decide whether you're able to detach it for replacement. If not, come back with more specific questions about your chosen strategy.