Can you retrofit an older riding mower to have an emergency shutoff?

yes run wire from ignition coil to bottom of seat use spings or whatever you not to make the wire ground to the seat pan when rider exits the mower this will automatically shut the engine down when rider falls off

to save yourself some time and cash just go and find a 2 wire seat switch off of an old riding mower and run one end of wire to coil and the other to common ground