A structural crack wouldn't necessarily be in the hull of the habitat modules. The ISS has a big frame (the Integrated Truss Structure) which supports the solar arrays, power distribution and heat radiators. If the frame is damaged, firing the engines to maintain orbit might be impossible.
Atmospheric drag reduces the altitude by about 2 km a month on average. Orbital boosting can be performed by the station's two main engines on the Zvezda service module
I hope it's all bullshit but if it's a tiny crack, but facing forward and up, the escaping air would push it down. I can't math it but I guess it would only be a small effect.
Decent rate doesnt seem to have changed but it will eventually deorbit if not reboosted like it regularly does. If there is a structural issue though, reboost might be risky. In that case we would need to bring back everyone and let it burn up