I was given a Yamaha DGX-620 for free, which has a broken screen and bad loudspeaker. This is an easy repair, it just involves replacing the screen and the speaker.
I bought the screen off Taobao from a domestic seller. This particular model seem to have improved upon the original design which use a heat pressed conductive rubber stripe.
I bought a pair of car speakers with the same size. These seem to be for Nissan EXA N13. The mounting ears needed some modification.
Not shown in this photo but I also applied RTV silicone to seal the mounting surface.