Try checking the battery terminal voltage during cranking before beginning to swap parts. It's normal for a 6V battery to drop to 4 1/2 volts or slightly less while cranking the engine. One thing that gets neglected when diagnosing a slow crank condition is the ground circuit. I like to use a large-diameter, longer than standard ground cable and attach it directly to a bellhousing or starter attaching bolt, then use a smaller gauge ground strap to the body and/or frame for light and voltage regulator grounding. The starter is by far the heaviest current draw item, and a big ground cable attached very close to the starter housing is a big advantage for good cranking speed.
Jerry
