Look a little closer- - - -there's a spring loaded copper (or maybe brass?) ground built into the vacuum advance unit. If you choose to use an extra ground wire, make sure it's a "high flex" type like the wire used for voltmeter test leads. The constant movement will cause conventional primary wire to fatigue and break strands very quickly.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!