I went through the same thing last year with my sons bike. The windshield had a yellowish "scale" on it. It was so bad that I thought nothing I did couldnt make it any worse!
I used Novis #3 & a cheap electric buffer I picked up at wally world. I used several clean bonnets. I started rubbing by hand with a soft rag but it was too much work.
http://www.novus-polish.com/index.htmI think I followed it up with Micro gloss polish...same stuff used on fiberglass tubs to remove fine swirl marks, etc. Whatever you use, start in a lower corner and test it first.
It was a very labor intensive job but it still looks clear and was cheaper than buying a new WS.