Sorry to hear about your rainfly. Question: has the window fallen apart (i.e. is there now a hole in the fly where the window was?) or is it intact, just discolored or faded? If it is completely gone and there is a hole in the fly, you can use a piece of plastic (such as tyvek or the clear painter's tarp material from a hardware store) and some fabric glue to manufacture a fix. If it is just not functional as a window then you may be able to at least clean off the sticky part of the inside and still use it.
In terms of the 'stickiness' of the rainfly, I encourage you to check out the above-mentioned thread about a solution of dish soap, warm water, and isopropyl alcohol to remove the sticky residue. Afterwards you will need to retreat with some of the product listed in that thread.
While do not have replacement rainflys available for purchase, you can always keep your eye out on our Used Gear site or at your local REI Store for a garage sale where you may be able to find a 'new' rainfly for your tent.
Hopefully this helps!