The Dolmar/Makitas are not my specialty but here's what I know. The stock carb will work along with the 7910 piston and cylinder. You can also modify it for more fuel. There is another carb that can be swapped in and I believe it's the Walbro Rwj-4. The 6421 is a really smooth and strong feeling saw. You should be happy with it as is! If you ever feel a need for a 80cc saw you can upgrade it and have a beast! Do a search for lithe upgrade as there are a lot of those going on right now!
Dan
Dan