Exhaust leak

[mrlmd1]

I also said contact cleaner. Sometimes the WD40, aside from being more immediately available, will clean it up altho it's temporary and can leave an undesirable film on something. Contact cleaner is the ultimate solution.
It could also be the starter solenoid, a bad corroded connection somewhere, a bad ignition switch, a bad safety interlock switch, and I assume you are not forgetting to hold in the clutch when you try and start it. Have you opened the headlight bucket to check if there are any frayed/shorted wires in there?
Did you try and apply voltage right to the starter (with battery cables) to see if it turns over? You checked all the fuses including the main fuse?
If you can find the starter solenoid, you should be able to test if any voltage is applied there when you hit the start button, and if so, if any voltage is coming out the other end going to the starter motor. That should give you some clue that the problem is before the solenoid, ie. in one of the switches - starter, ignition, cutoff, sidestand, or clutch.