I had my cylinders rebuilt for 80 bucks. And picked up Wiesco forged pistons on ebay for 80 bucks when I did mine. If youre patient.. You can find the deals. But that link you found may be a better choice if youre unfamiliar with engine machining. Depending on the cylinder wear, depicts the correct pistons to buy.
Stock 650 piston is 76mm. If I remember correctly, they go in .5mm increments. So depending on the cylinder wear, would dictate the correct piston to purchase. (stock is 76mm, biggest available is 78mm) That link you posted would take out all the guess work.
In that link, they rebuild a set of cylinders and send them to you with the correct pistons. But they need your old cylinders so that they can rebuild and sell to the next guy. Its just like a core charge when you buy an alternator, or a battery at the local auto parts store.
Note: forged pistons are better. Forged is a process of how they are made. Most race skis have forged pistons. Stock pistons are simply a mold of aluminum that they pour, then machine the piston. Forged is a lot trickier of a process. But its a much stronger piston.
If its for sale, then dont worry about forged pistons. If youre going to keep it... Buy Wiesco.