Its probably PETG, like this

http://www.curbellplastics.com/sign-pop/petg.html

If you can get a scan of the missing pieces, you can make a pretty good replacement (at least something to have until something better comes along).

I did this for a missing plastic and it turned out pretty well, with minimal effort. You can see the difference with the game off (see photo), but when the piece is lit up it is almost indistinguishable from the stock pieces.

Steps summarized:

1. Print on normal paper, on an inkjet printer and adjust the colors as closely as possible. Use a paper that is not too heavy, so light passes through--20 lb paper or so, but you can experiment with different weights.

2. Roughly cut out the plastic, and fine tune shape with a dremel tool. Leave ~3/16" clear 'edge' for the tape to have something to hold to (see step 3).

3. Tape the image to the underside of the plexi with clear packing tape. Trim excess tape.

4. Drill holes, install.

IMG_0511.JPG