If you are transporting it yourself, it's not too much trouble. Just have all your receipts showing you bought it, and you'll need to sign some paperwork at the border. I hear it also helps if the tractor was made in the USA. 

If you are having it hauled by someone else, you're going to want to get a broker to handle the paper work at the border. It's better to have one before the crossing so that everything's in order, otherwise expect a wait while one is found. Farm equipment should be tax exempt, and a broker will make sure you don't pay more than you have to.