I am looking for an ignition switch for my Huskee 18hp riding mower which turns the lights on/off while the mower is still running. I still have the owner's manual for it but all the parts dealers say that the mower is so old that the part numbers are outdated. The one's they keep giving me work, but the lights stay on all the time. Whereas the one that originally came with the mower, you could turn the key one click back while the engine was running and the lights would come on. One's they give me does not do this because when you turn the key one click back, it shuts off the engine. It is an old mower but still runs great. Has a 5 prong plug which plugs into the ignition.

Here's what I would do: add a toggle switch to turn your lights on/off. This should be a simple procedure, of course, I'm a retired Electrical Engineer. You just need a Single Pole Single Throw, which you can buy at Radio Shack or you corner auto parts store.
You should be able to mount the switch on the panel that contains the ignition switch by drilling a hole to mount the switch.
Determine which wire provides power to the lights, then you may have to add some hook-up wire to finish the job.