The fan not working, may be the best news. You will have to find a safe way to get up there and have a look at the structure and if you get up there don't forget the camera. Your best bet is to talk to the local truss company. Take a few photos of the house and measure everything that you can see before you talk to them, then they will send you home to measure some more. They should be able to give you trusses that sit on the two wall and put very little weight on the old roof. Don't forget to tell them you want to raise the walls at the same time. My first thought was you could run it to the peak but that won't work with the other roof so you will need pictures of the inside of that roof too.