The main engine ground deteriorating was a suspicion for the starting issue I was having as well, mine turned out to be a combination of a cooked battery and a thoroughly fried starter solenoid, one possibly leading to the other. Was worth checking though anyway.
It looks like the main engine ground on the 650 is somewhere on the back of the block from the clutch side, the routing is shown in the K4 service manual but it's not super clear, likely to be easily traceable from the battery though. I would recommend removing the main ground wire, thoroughly cleaning with a wire brush and re-installing with a dab of di-electric or corrosion preventing grease. An electrical ground secured with a steel bolt into aluminium will always fur up over time and increase in resistance. A damn good clean up might prevent the need to add an earth.