And what shape is the starter in? Is the switch (on top of the starter) burnt to a crisp? Does it need new brushes?

If the engine is turning over slowly, check these items:
1. Charging system
2. Battery
3. Timing and ignition
4. Voltage drop, both for connection loss, and how much does voltage drop when starter engages.
5. Connections
6. Starter

Fred
52 3600
69 C-10