Theoretically, you could probably take all the inner workings out of a cheaper digital piano (say a Yamaha P70) and fit them into the keybed area of the upright. You'd just need to do some wood working around the inevitable gaps created. Also, you would need to make a control panel for the volume sliders, and ports for the power input and headphones. I would suggest putting the power/volume to the left of the keys. I'd also suggest under the keybed for the headphones. There shouldn't be many electronics inside a simple digital piano. The digital keybed of a Yamaha should come out all in one piece. While you're at it, you might look into upgrading the internal amp and putting in some better speakers. The sound board already in the piano might help out the tone.
If the digital piano is slim enough, you might could mount it into the piano without modifying the digital piano itself. However, the piano would more appear to be a conversion with it's plastic case. I've heard of this being done before, and in the right hands is very possible. In fact, it sounds like a fun project!
Les C Deal