You'll have to search. Search function will keep you busy with that for a few hours. Wiseco puts a window in there. I think Stewart is making it bigger, with dyno-backed precision. Its all about increasing power. Putting a fresher fuel charge in the cylinder and getting the exhaust to exit completely, make it run more effeciently and increases power everywhere. Its just one of the many mods that add up to the fastest dirtbike in the world.
Pull that cylinder. Then you will know, if you need a base gasket or a top-end kit. Wiseco has a great kit that comes with all gaskets, rings, wrist pin, needle bearring, piston with window. You really have to see cylinder to be sure the coating is OK. The coolant can cause alot of damage if it is leaking inside. Its better to have it apart for a short time, than to destroy the whole motor putting off the repair.