Rather than immediately trying to restring the entire instrument, try this. If it has an iron frame you may be in luck, if the frame is wooden, you may never be able to get it in tune without regluing the entire thing. The technique: one by one, detune each wound string until it is going "boing" (like in a cartoon) and you can physically pull the string about two inches (like pulling on a bow and arrow). Detuning allows the windings to compress and squeeze out all the garbage they have accumulated over the years. Now pull out the string as far as you can and let go like you are shooting an arrow. This will dislodge most of the crud that has built up over the years. Snap each string several times and the string should be quite clean of debris. Examine the string and brush off any crap that is still attached. Retune the string and move on to the next.