Hmm.
I'm sure someone else has thought of doing this before.
My main goal is I don't want to have to remember to always have the key with me.
I'm just curious, but could you just leave the key in the ignition? If you would be worried about someone starting it with the key left in, couldn't anyone start it with the push button switches also? Or were you planning on hiding them from view?
Another way to solve the the problem of getting to the tractor and not having the key, is to use a magnetic key box somewhere on the tractor, like under the seat or under the hood.