You could put down a new plywood floor using shims over the existing floor to level it. It would probably be just as easy to do it the right way, by removing the old floor and installing new, level floor joists next to the old ones. Or you could try to raise the low end of the origional floor joists with a jack and use new metal joist hangers to fasten them in place. You might be able to do this with the existing floor in place, if there are no walls built on the floor. Other wise, you would be lifting the wall & in turn the roof. Not a good idea!