[wga]

As mentioned the cable to the starter should be of 00 or 0 gauge copper, properly crimped
ends. Check that the cable is not frayed. The end crimps should not have bare wire visible, which leads to corrosion and resistance. Starter can be removed and cleaned for good ground connection. If the starter is in need of rebuild, also consider a new gear reduction style, for about the same $$ or less than a new / rebuilt stock starter.
The motors on these draw less amps and use the gear reduction to multiply the torque applied to the flywheel.
Optima battery is the way to go as well. I ditched the lead acid battery.

Had the same problem on my 49 when I first acquired it, 22 years ago.