If you never intend to move it you may be able to cover the roof with thick plastic and seal the sides to stop further intrusion. Lap it down a couple of inches on the sides, If the roof is collapsing already you may have to put a couple of sheets of thin luan up there first and secure to to whatever is left of the existing joist. Then seal with both a heavy tape (gorilla) and maybe even brush a coat of a sealant over it.
This is not pretty, and it will not stand up to any movement, but it should stabilize it depending on how badly its collapsing.
I have helped Hurricane victims make temp repairs like this many times, and thats all this is,,,, TEMPORARY.
Inside if you have a lot of water damage, mold and insulation become primary issues. If you cant seal off from them, scrap the whole thing.
