This is a fairly common problem with houses designed this way. I am doing the same repair in a couple of weeks for 1 of my customers. The problem you may find is that the duct may be rusted away to nothing. Repairing the pipe should be fairly easy.
You will need a jackhammer for the floor and then some PVC and a couple of banded couplings unless the leak is at a fitting. You really won't know what you need exactly until you get to the problem and see the extent of it.