i like the stock 13/40 sprockets for "quickness" in the tight woods trails i ride,
others like to jump to 14/40 for more top speed in more open areas
do both sprockets and chain together, if any are worn, it will wear the other new pieces
i like these myself from "forward powersports" and "racedriven", they come in a few different sprocket size combo's................
more power/o-ring
Yamaha 88-06 Blaster 200 Red ORing Chain Sprocket Power | eBay
more speed/non o-ring
YAMAHA YFS200 200 BLASTER BLUE CHAIN & SPROCKET + SPEED | eBay
ken will tell you if and how much your cylinder needs to be bored over, if at all.
over boring adds no extra power, that will all come from the porting and head mod