2 smoke spark plugs can look pretty clean and be fouled. You need a handful of spares 'til you sort it out. My brothers bike had a problem with the kill button. It was either shorting or the spring inside broke. Turns out it was the intermittent gremlin we were looking for. Try unplugging the button if you can or somehow disable it. Check your spark and ease of starting. Have a look at the petcock and see if it leaks when off. That will fill your cases with gas and make for a hard start. Make sure there is no crud or water in the gas. Are the head bolts/nuts snug? Keep trying and report back. Good luck.
1991 Honda XR628R
1978 Yamaha XT500