If its 200 years old, it won't be a gangnail truss. Is your property listed? If so, you will need planning permission (just thought I'd mention this 'cos of the age). I would have thought you could still splice a timber on to it, but due to the age of the timber, it could be tricky. You need to find a really good carpenter who knows what they are talking about. (Just thought, try phoning the National Trust, they may be able to help track one down, as it is an old building.) Otherwise, you could just try looking in Yellow Pages, or even try your local timber yard (not Travis Perkins or Jewsons or whatever, a proper timber yard). They will know who the good carpenters are in the area.