i doubt spilling fuel in the oil tank would have caused it although it will make it run slightly less oil in the engine as it would dilute the oil .all depending how much went in there ?it sounds like the piston has had a little nipupeither due to your weak oil mix or just because it was its time to fu*k up.there are lots of reasons they seize a loose circlip/weak mixture/lack of oil/broken ring peg or broken piston,shattered reed.the ignition timing should be ok as its cdi but the only way to tell whats up with it is to take the exhaust off and the right hand(black) fan cover if its air cooled and undo the four 10mm headed bolts that hold the head and barrel on lif off the barel and head and have a look at the state of the piston and barrel if it scored up then it will need a rebore and or a new piston too and you need to determine what has caused it also or you will be back at square one again.
adam bike doctor, December 2009