When you say that you checked the spark plug ... did you put in a new one, do a proper plug chop, and then check it? Or are you using the old one? If you're using the old one, it'll be pre-browned, and you'll never know how well you're running.
Yes, you do actually have to buy a new jet. They're these tiny little pieces of metal that have a very very small hole drilled in them. The gas flows through that hole. The bigger the hole, the bigger the size (number). When you buy a 57 jet as opposed to a 55 jet, you're getting a jet with a bigger hole in it to allow more gas into your mixture. I don't know what kind of carb you've got on there (although I'm assuming it's some breed of Dellorto.) If you can fill in what kind it is, we can probably tell you where you can buy a new jet.