Its a rather common issue with most of the vintage gear that all of the backlit LCDs fade with time or the backlight dies altogether. Same fate met this Kawai unit, the step up converter was just fine but the backlight foil just went past it’s useful life.
One way to deal with the problem is to replace the foil with a new one, the other is to use LED backlit display.
I have in this case opted for white on black LED backlit LCD. The exact part number is LCD-AG-240064A-FHW K/W-E6
It wasn’t without issues. First of all the cable is connected upside down. So you’ll have to put nr1 wire on nr20 position. Secondly the display quality is so so. Far from perfect but it’s fair enough.