I know a number of people have used the Mastercom player tracking display to display messages.
They read the chip into a binary file, use a hex editor to change the message and burn it back out.
Here is an old old thread on this.
[size=78%]Bally Mastercom-220 message change (newlifegames.net)[/size]Now if your just trying to get some messages displayed.......
You can get a Mikohn ChamII+ (note the plus) standalone progressive display. This can display a progressive amount and messages.
Typically this is used behind your top glass but looking at the PIC yours does not have a cut out for the display.
I have 60 different game kits for my S+'s (both in 16" and 9") sizes - I never did get a round top.
Finding the glass and reel kits is hard enough but finding ones with the display cut out is another challenge all together.
What I did is to take blank player tracking plate and a Dremel and I cut the display into that. This way I could have a progressive for all of my game kits without modifying them.
I have also gone overboard and mine is a linked progressive between 3 slots and my PE+ with a display in each, accompanied by the 4ft sign over the slots and the 18ft sign over the bar.
I have also seen people get a small scrolling display - typically used for cars - ebay is loaded with them and they mount these in their player tracking plate.
If you want to get fancy you can get a small backup camera monitor (takes HDMI input) and connect that to a raspberry PI or old laptop (albiet old laptops rarely have HDMI) and display whatever you choose.
I have a framed (aka from Michaels) LED picture frame that I use for pictures of friends with the fake Giant Cheque for big hits on the slots. This is mounted on the wall not in the slot.
I run my slots on tokens so no one loses anything or wins anything but that is beside the point. I do roll my tokens so people get the sense of loss when their roll runs out.