This is going to be short and sweet but important: someone has created farm kid valentines. Check them out. They are darling and easy and I'm pretty sure your farm boy or girl will love them.
Print them on white paper for a boy, pink for a girl. Done. The only thing they can't help you with is if you are a die-hard red equipment family. And I'm sorry about that.
And if you're feeling craftier? Check out all that Pinterest has to offer for tractor Valentines. There's something for any manner of crafty skill...or time commitment.
I would be all over those tractor boxes, but my farm boy will be spending his Valentine party day at the National Farm Machinery Show. I thought he might be a little disappointed to miss his party but no. "Eh. That's ok," he said to me. Zillions of square feet of equipment will do that to a boy.