How do I upgrade my disc brake rotors from 160mm to 203mm?

I have a 2009 Kona Stinky Six free ride machine. My front end has Marzocchi 55-RS forks with a 20mm thru axle mated to an over sized FSA 1.5" headset . I currently have the stock Hayes Hydraulic Stroker Ryde V7 brakes with 160mm (6 inch) rotors. I bought this bicycle because I have been looking for a bike that doesn't fall apart and get wobbly wheels while I'm riding on flat pavement. It's like that due to the fact that I'm 6'4" 340 lbs. This bike is perfect for me except for one thing: If I go down a steep hill the brakes hardly work at all. Riding on the same hill, other people on my bike stop on a dime. We got some steep and long hills around here and I'm heavy.. 160mm/6" rotors aren't even close to stopping me in time.How do I upgrade from 160mm to 203mm rotors? Where can I purchase the rotors? Where can I purchase the upgrade components? I would like to keep my stock brake calipers, if I can. I appreciate the help, thanks for reading. Scott Harriger, Pacific Grove, CA.