One more thing: If the PU has not deteriorated too far, that is, if it is sticky and stinky rather than dry and cracked, you may be able to restore it temporarily - for a few uses - by throwing it in a medium drier for 15 minutes.

And, if you decide to replace the floor and fly, consider the double-coated silnylon from:
http://www.americanhomeandhabitat.com/products/Silk-iene%20Silicone%20Coated%20Nylon%20Ripstop.htm

It is less permeable than simple silicone permeated nylon, so there is less splatter-through through the fly and less seaping through the floor.