One question I often get from customers is weather or not rotted decorative wood trim or structural members can be repaired instead of replacing them. With regard to structural, often, for instance, a rotted floor joist can be married to a new one if the joist is solid along part of its length. The concern though is that it would continue to deteriorate and the fastening hardware no longer able to provide the necessary strength. With regard to decorative trim such as cornices, or hard to remove pieces, customers often ask if there is a way to repair the wood without removing it.
While not perfect, there are products out there; epoxies that can seal rotted wood and other products that can kill fungus and harden the wood. Here is a link to a website that has a product called CPES which does all and more of what I described above. Its often used to restore boat hulls but works well on home restoration projects as well. I have used it to repair rotted window sills.