Seems your heading in the right direction.
You say you've changed the plugs, but even new plugs can be duff. Take the left plug out put it back in the HT lead and lay it against the cyl head. Give the kick starter a few swings and see if you can see a nice fat spark. I would also try swaping the plugs over as well and see what happens then.
Good luck and I hope I'm not teaching me granny to suck eggs