This post reminded me of something I saw today. Went to Walmart to grab a couple things with the girlfriend and my daughter. As we are walking into the store there is a nice Harley parked there with the helmets on the handle bars like normal. Something on the seat grabbed my attention and there are the keys to the bike. Just left laying on the seat in the parking lot unattended. So either he is very trusting or he was hoping someone would steal it.
As for the tractor, you should be able to buy a new lock at an auto parts store and swap it out giving you a new key that would be different then anything else out there. Then again if you install a kill switch you can have it even more secure as even with a different key it can still be started rather easily. A kill switch in a hidden place would do a lot more for safety and security.