A common problem is loss of the ground at the handlebars. The starter button needs the handlebars to be grounded, in order to send that ground to the starter relay. You can try connecting a temporary ground to the blue/white wire terminal at the starter relay..................that is the same as pushing the PB. If the engine cranks, then you are missing the handlebar ground.
The 81 year also has a Starter Lockout Relay. You need to have a ground from the neutral switch, or the clutch pulled in, to allow the starter relay to work. If you want to by-pass the Starter Lockout Relay..................find the relay and add a wire from one red/white to the other red/white. With the transmission in neutral, if the engine cranks with the by-pass wire in place, it means the relay is defective or the neutral switch is not providing a ground on the light blue wire to the SLR
Wih the transmission selected to any gear, the clutch switch should send a ground to the SLR on the black/yellow wire, and allow the engine to crank.