This could be simply a weak battery.
Measure the voltage across the battery during cranking. If its below 9.5 Volts, its a weak battery. Try a slow recharge. It might come back.
Because battery's provide so much energy to the solenoid and starter, to test for power loss to the coil during start, clip one lead to the battery positive, and the other lead to the coil positive.
This will show voltage drop along the whole path to the coil. Because the points are opening and closing quickly, the meter's needle may fluctuate. A digital meter may be worse. Because of the pulsing nature of the current flow through the running coil, this is not a situation that lends itself to measuring for voltage drops.