If you haven't already, try polishing the underside (it is like the lower field on Congo, right? wrong?) of the window with Novus 1 or 2.

Make sure first that it's CLEAN on the underside, as wiping it down. I can't imagine that anything other than water would be needed on the underside, since any crud on it would just be metal dust lofted by the heat from the bulbs, like the crud on any insert.

Assuming you've done this and come to the conclusion that the upper surface is the problem, you'll need to polish it up - polishing clear things so they come out transparent is tricky, as I'm sure you know. You might be better off trying to find someone selling a brand new repro - it's not a hard part to fabricate with a laser cutter, so i'd be stunned if it wasn't available. As usual, trying to find it in marco's web storefront is hopeless.

I wonder if you could use a car headlight cleaning kit to clear it up a bit...Anyone ever try that?

http://www.amazon.com/Turtle-Wax-T-240KT-Headlight-Restorer/dp/B001TI5IIQ/ref=sr_1_1

Car headlight kits are for removing oxidation from Uv exposure. Doubt that's case here. I polished mine with a high speed buffer and rouge and it came out clear. Bother the window and decal are available stateside but I'm on my phone so can't look up right now.

I have seen this part available from a supplier. Its been in the last week but I have looked at so many sites...I cant remember who had it.

Pinball decals used to have them but out of stock. Ill look around.

Out of stock at MAD Amusements but might want to call those that say it's out of stock to see if they can't hook you up. Bit pricey but what can you do?

http://mad-amusements.com/product.php?id_product=389

$75 at little shop but out of stock there too..

http://www.littleshopofgames.com/Games/cftbl.html

Looks like down under is the only place that has them in stock. Odd...they were just available at baa 6 months ago. Must be all those congo purchasers snatched them up

http://www.pinballspareparts.com.au/index.php?crn=272&rn=3343&action=show_detail

I would try polishing with Novus2 before buying a new one.

Novus 1 is a lighter polish than Novus 2, so if 2 is making no difference, 1's not going to help. 1 is for a 'final' polish. Novus 3 is more agressive. What are you using to polish. That plastic I believe is a form of plexiglass so just hand rubbing with a rag isn't going to generate enough friction/heat. If you google a bit, you'll see that many have had success using a high speed buffer like Treasure Cove's kit or an auto polisher. I did mine with a high speed buffing wheel and it came out great.