The TI-83 and TI-86 series calculators were Z80 based.
Apparently they discontinued those in 2004. Huh.
Maybe the m68k would be a better target? Actually if the OP is serious about this project, I would write base kernels for Z80, m68k, armv7, and RV32i. The last one isn't widely available, but has the advantage of being both modern architecture and allowing open-source specs for how to construct one from scratch.
