The seat is not replaceable. It's pressed into the carb body. Good news is that there is rarely need to replace the seat. The needle has a rubber tip so there is very little wear that can occur on the seat. The needles on the other hand are prone to wear/damage. I won't speculate on whether yours need to be replaced though.
One thing you can do to the seat is clean it. I use a piece of gray scotchbrite. A piece that's sort of worn down already works well because the grit is diminished some. I cut a strip and then roll it up into a tube so to speak and shove it into the hole in the float needle. Then twist and press until it reaches all the way to the bottom, where the needle touches. Spin it around a few times and pull it out and look to see if the metal is bright and shinny on the bottom. If it is clean out the grit and call it down, or go back for round two.
When sourcing new needles you need to understand that most aftermarket brands, such as K&L, are inferior to the original Keihin parts. You can purchase the real Keihin needles from Jets R Us though...
www.jetsrus.com/individual_parts/018_880_su.html
Instead of just going into the carbs and replacing selective parts you would be much better off to fully tear down the carbs, soak all the hard parts in carb dip, replace all the various soft O-rings, and then reassembly the carbs. Do this and you will only be into the carbs ONE TIME. Take short cuts and all bets are off.
