ok: go to the wiki and learn a little more about cylinder kits
but the break down basically goes you are getting a larger displacement by switching to a bigger cylinder size. your stock newport is 49cc (cubic centimeters) if were to put this into car terms you have a .049L engine hehe since most cars run off of liters 5.0L(mustang)=5000ccs so in essence you are increasing the size of your engine. this boosts horse power output, and more power=more speed.
the bigger carb allows more fuel, and more air to come in, this equals to more action in the cylinder, and that means more speed again, the high compression heads increase the compression inside the cylinder itsself. and more compression again equals more power.
then you get into new exhaust systems. these can increase low mid and high end power depending on the pipe. some increase one, others are good at two. no pipes are equal across the board, or thats just stock..lol
jets are for tuning that added amount of fuel/air when you change to a new carb, or change filters, or run without one at all