If you have sparks at points then that verifies the ignition supply side ( switch barrel, kill switch etc) are OK as you are just testing via the earth route.
If the HT leads are in any way suspect with age or condition, just try getting someone else to crank it while you look under the tank in the dark, if they are leaking you'll usually see it sparking across somewhere under there.
It only needs to leak on one lead, for example #4 for it to take out both 1+4 together.