This is what it looks like at the back.
Wires of interest are the Red and Blue/Red-trace.
View attachment 1799180
So from the front like this.
View attachment 1799181
So you turn and leave the key on (in it's off position it shorts out the ignition coil) which provides power to pin 1.
For you (or the inverter, or a remote, or a switch etc. ) to start the generator simply connect pin 1 (Red, power) to pin 2 (Blue/red.)
The generator then starts after a few seconds on it's own and remains running until you break the connection.
As easy as that.
The plug is called a GX20, 5 pin.
The key being "on" simply puts power on the pin and doesn't seem to draw anything at all, not anything I could measure anyway.
I have no idea what the Red/Yellow-trace wire does.
The negative/earth is not needed if you just want to start/stop the generator.
I am having trouble finding a plug though. I can find GX20's in all different pin numbers except for 5 at the shops around me. Looks like I will have to order online somewhere perhaps.