The question is what changed? Did you particularly violently miss a shift? Did you do anything out of ordinary (that's sort of a moot question the way you ride
)? Did you notice a backfire right before it happened?
My initial bet is that the cam chain skipped a tooth. (I was writing this while thinking about it, all the stuff to do is good, some is easier to do in order, but you could skip right to checking the cam timing)
First, remove the spark plug and inspect it for deposited metal. Next, check the oil by draining it and straining it through a paper towel. Take out the filter and cut it apart and look in the pleats for metal. Same with the frame screen. If you find metal, it's time to take the motor out of the frame and start dissecting it to find the source. Fine swirly metal dust is not good, gritty chunks of metal are bad, big pieces are real bad.
Next, check that the motor spins freely - remove the crank bolt port in the stator cover and turn the motor by hand using a deep socket on the crank bolt - it should turn smoothly with light drag all the way through. If it has any more than light drag, or if it feels gritty in any spot, you probably have a bad piston.
While you have the plug out take off the timing inspection port on the stator cover, attach a timing light to the plug wire (be sure that there is a plug in the cap, and grounded so the coil energy has somewhere to go) and crank the motor to see if the spark is happening near the timing mark. If it's off, check the key and keyway under the stator. I have a stator removal tool that you can borrow if you need.
Next take the valve cover off, and see that everything is right up there -- check the valve clearances, which won't necessarily be the actual cause, but will definitely show a bent valve as having more clearance. If there is NO clearance, that needs to be fixed, but do that later. Check the valve timing - set the crank to TDC (there's a mark on the flywheel) and see if the slash-marks on the cam chain sprocket line up. If not, rotate the crank 1 full turn and check it again. If still not, the cam chain skipped a tooth. If that's the case, you'll need to replace both the cam chain and the cam chain tensioner. The auto CT works, despite the DRZ urban legend (only affected the first couple years), but a manual CT is cheaper than OEM unless you can get a used one.
