[Accelo]

Are the ground straps, from the firewall to the valve covers, still attached?
I would run a wire from the + side of the coil to a bulb and install it where I could observe the bulb from the drives seat. Ground the bulb to a ground on the firewall and see what happens to the circuit when the clutch is pushed in. Then do the same thing with the ground on the motor. Seems the grounds should be fine because the starter works but need to start somewhere. One has to assume your have compression and fuel. That only leave electrical. I would also pull the engine harness plug from the fire wall and confirm it's copper terminals are in pristine condition. Yup that's a good one!
Cheers