I purchased some MAX7219 based 7-Segment displays and reworked the case "small display" model to hold the display. The digits are larger than on the original and the color is wrong but I think I can live with that. 

I had an ESP32 on hand and have decided to use it for this project. It's total overkill for the straight calculator version, but will hopefully have the horsepower to allow me to do some cool stuff for the "Next Generation" build. The display's 3-pin SPI interface leaves a lot of I/O pins available for other stuff.