I'm thinking this particular bike has a systemic problem. Maybe something like extended exposure to corrosive atmosphere or simply a previous owner who was an assinine amatuer mechanic.
The repair manuals provide a step-by-step process for troubleshooting. My experience is that most people lack the self-discipline to read and follow the directions given. This is a penny-wise and pound-foolish mindset. Another common distraction is the desire to hear the engine run. "Just get it running" becomes a mantra. When I competed in auto mechanics competitions in high school this thinking was drummed out of us. It doesn't work, and in real life pushing for an immediate engine start is not only a waste of time, but puts other systems in disrepair at risk for further damage.
My favorite electrical system diagnostic tool is a 12 volt buzzer from Radio Shack mounted in a piece of PVC water pipe. The pipe provides protection from drops and bumps and a place to securely attach leads with some means of stress protection. It cost $0.99 years ago, but I'd expect a similar product would be under $5 today. A Velcro strap holds the device securely to my wrist.
With the buzzer and a schematic I can start at the battery and trace any circuit to any component in a matter of seconds once access to the wiring and components is obtained. I use a very systemic process that adds each stage of the circuit until a fault is found. When the fault is found, the last stage added is ALWAYS the problem. Something like this:
Battery - post to battery + post. buzz The battery is good.
Battery - post to cable end. buzz The connection is good.
Battery - post to other end to other end of cable. buzz The cable is good.
Battery - post to eye on wire to fuse. buzz the connection is good.
Battery - post to fuse holder. buzz The wire is good.
Battery - post to other end of fuse. buzz The fuse is good.
Battery - post to far end of fuse holder. buzz the fuse holder is good.
Battery - post to junction box. buzz Wire to junction box is good.
Battery - post to hot side of switch. buzz Wire to switch is good.
Battery - post to controlled side of switch. buzz when switch is pushed Switch is good.
Battery - post to wire from switch at selenoid. buzz when switch is pushed Wire from switch to selenoid is good.
Battery - post to control ground of selenoid. buzz when switch is pushed Switching circuit of selenoid is good.
NET: Nothing wrong with primary start circuit. Therefore, failure to start is in secondary (high amperage) start circuit.
If there is a low-conductivity point, the buzzer will have a weak tone. That's a dead giveaway the last point added has a loose, broken, or corrosion issue.
Point is, diagnosing electrical problems is best approached with a systemic process that eliminates functioning components as sources of the problem. Instead of asking "What is bad?" ask "What is good?" which leads to a process of elimination.