It's a common problem with them. You can buy parts on eBay. The display driver is relatively inexpensive however the display itself is over $200 and going up in price.
https://www.ebay.com/itm/Display-for-Narco-MK-12D-D-P-N-50401-0001-/131157812864
I just replaced the entire avionics stack in my Cardinal when the display in the Mk12D started to go out (again). The King transponder was dying too. The second Mk12D was fine (never had a single issue) but with the entire panel of mid eighties vintage, it was time for a complete upgrade. Glad I did it. The capabilities of the new equipment is like a magic in comparison. The pilot is now the weak point in the system.
If you want both Narcos I pulled (use one for parts), I'll make you a heck of a deal. But if you can afford it, I recommend biting the bullet and getting something newer, lighter, more reliable.