You are right, it definitely won't work without that. The spring is likely broken.
What engine is it on? I only recall seeing external mags with a gear that engaged with the camshaft, no direct couplings.
All model L Gravely's had an external Magneto driven off the exhaust camshaft, the mag was coupled to the cam shaft via a round phenolic spacer, each end had a slot 180 out form the other. One end of the spacer fit on to the magneto, the other onto the camshaft. On the camshaft end there was part that would engage that round phenolic spacer, there was a means to adjust that part to specified clearance to the magneto.
Every magneto that gravely used on the L was equipped with an impulse mechanism. The impulse was only active at low rpm, which would in turn wind up a spring attached to the rotor, when the spring let loose it spun the rotor at a much higher speed then the engine, this quick Snap! action would generate the necessary spark for ignition. If the Magneto Impulse is inoperative it is highly unlikely that enough spark if any would be generated for ignition.