Hmm... Looks like the same apron and subsequent art was used on a few games from the same time frame from Williams. So far, I've seen that Wonderland, Skywat, and 9 Sisters all used the same apron design and the same artwork, which seems to be a large paper overlay that's been adhered to the apron. If you get on the IPDB.org site and do an advanced search for other pins in that same era, you can probably find all of the games(using the pictures) that have that same apron design. Then, take the list of games and enter them in here on Pinside to see the people who own those titles and, if they're active enough members, see if you can PM them to send you pictures of their aprons with the different measurements you'd need to recreate one. Doesn't seem like it'd be too hard to have one made, but having one on hand would be a massive help for measuring and replicating. Heck, even if you could get someone to make a pencil or charcoal scuffing of the edges and outline for you and have them send you the paper, that would be fantastic. Seems like the thickness would be identical to the inner siderails and arch, so you'd already have that measurement done for yourself, if those are intact on your game.

Here are some links to CFH's site with pictures of some of the games that had the same apron:
http://www.pinrepair.com/wmswood/9sister.htm
http://www.pinrepair.com/wmswood/skyway.htm
http://www.pinrepair.com/wmswood/wonder.htm

As for the artwork, if you can find a nice straight-on shot of one with all the little bit under the Williams logo on the bottom right in focus and a correct height and width for me, I can churn that out for you super quick and you can take the file to a local printer and have them print it directly onto a vinyl decal and you can apply it on there once the apron is remade.

Between the PinRepair site and IPDB, I think you should be able to find several good pictures of what the apron was supposed to look like originally and hopefully there's someone here on Pinside that has either that game or any of its brethren that have the same exact apron that you can get in touch with to get an imprint of it and send you that piece of paper. I think it's totally doable.

See Peter "Inkochnito" Koch‘s site at http://www.pinballrebel.com/pinball/cards/

A pdf of the apron is listed under Williams (“Apron cover SC-A”).