The yellowing is really only noticilble in the 100% Translucent "Window" areas...
What I would probably do... would be to cut the window areas out... and then glue that plastic, on top of a brand new piece of plexiglass.
(Or slightly oversized Lexan... making it a bit less likely to crack on impact / airballs.
So long as you have the bolts / screws long enough... for the extra thickness. Otherwise, you may need to either order some different longer hardware,
or find something equilvilant. (Or have something custom machined)
Sadly... most Repos out there are very expensive... and the person doing the color work... appears to lack the eyes capable of seeing / discerning the full color spectrum... and so... we get a pile of overprices garbage, that doesnt match either the existing parts / playfield... nor the original games color scheme in grand total.
Hope you are at least temporarily planning to put down at least a new piece of mylar... otherwise that ball will Destroy any / all work you put into this... in a mere month of play. All those fine crevices, will be soot black and stained even worse than before.. and the ball will start tearing that paint off the field in no time flat.. despite any wax put down.
For missing paint areas... I once saw a video of a guy whom used scans / photos... cleaned them up in photoshop, and then printed them on Water-Slide decal paper... using his inkjet printer. (The stuff they use on those plastic model kits) Because they are thin, they are often semi-translucent... so he had to stack a few layers worth... to get certain areas fully opaque. However... if the area could be pre-painted white as an undercoat... that might reduce the amount of layers needed.
Also, if you do use them, you will Need to clearcoat the field.
Finally... when all back together... before you try putting wax down... see if you can find F-21 (By the company turtle wax). Its WAY better than wax. Slick as all hell. Glossy. Has Ultraviolet protection in it... to help reduce sun-fade, covered over small pits and scratches, and the game will play like Factory new. Which for most peoples older machines... means anywhere from a 15 to 25% ballspeed increase.
No dirty wax being pushed into every nook and cranny anymore either... which is the Ultimate clincher.
Re-Apply every month or so... depending on use. In the busy arcade... I was wiping all easy to get areas down every 2 to 3 weeks. 2 weeks ... still looked like new. 3 weeks... started to see signs of some cloudy carbon sooting. That was of course... being played from like 9am to 9pm... 7 days a week. With wax (before my discovery) ... Those machines only lasted a week before there was crud on the field... and in 2 weeks... the soot and dirt was at times, covering over details on the PF... and that was After a completely full shop job - with all ramps and sub-ways cleaned.
After switching to F21... I didnt have to do any full shop cleanings... but maybe once every 6 months to a year... and not much was actually pulled up. It was mostly just to check for mechanical issues, and to verify the results. Two Pins in the store... were using F21 for like 2 years straight... and not a single issue. Sales were totally boosted, due to the games looking spectacular... and playing smooth as glass... and lightning Fast.
No more black soot on white rubbers. And no more crud stuck in between crevices on clear star-posts. What a godsend that was! =]
Congrats on the Pin. Nice to see some Pinball news again. I really miss the hobby! :[