Yes, you can fix the ribs. Usually what I do is make a new rib (either tracing the rib from the plan or just fitting in a new rib and cutting/sanding to shape). You need to rib(s) in place, the spar needs it as it support. If you just try to cover the spar, it might flex in flight. This type of fix is pretty easy, just take your time to ensure that everything is the same size. Thickness of the balsa rib is usually not an issue.