There are "homebrew" games legal available.

Most of them for 2600 because that system is most challenging.

A 2600 emulator may run fast on Newton OS if properly recoded but may

look very different in grayscale and input is a bit difficult.

NES emulation may require a good optimizing compiler even

for the 2100 and perhaps some assembler coding.Things like a SNES

seem not very possible to emulate,because the ROMS need to load somewhere in your RAM.A 16bit Palm OS device may be better to emulate

because the Newton is similar but much faster and has a bigger screen.