Hello U,
Do you have a multimeter and understand how to do a voltage drop test on cables and wires?
Try using your jumper cable and go from the battery negative to the engine block. See if it will crank then.
At this age, the ignition key switch is also suspect due to years of use. If you replace it, it has to be an exact match with the correct letter codes next to the each of the terminal pins. There are several switches that "look the same" but are internally different.
You will need your PIN label number information off the white label on the frame (above one of the rear tires usually, or on the side of the front chassis)
Last but not least, do not overlook the 20A main fuse. they get corroded in the pins. Usually, you can pull the fuse and make sure it is good then reinstall it. Do that a couple of times to scrub the pins and make good contact.