Just a few thoughts:
If the ground strap is old, it could be corroded internally. I had this problem on a tractor I had. Even after converting to 12 volts, it was a pain to start. When I replaced both cables, it would crank so fast that it would make oil pressure!
Another thing to check for a slow starter is the switch contacts on top of the starter. I had to replace the one on my '46 last year. It was getting so it didn't want to crank and a new switch was all it needed.
My final thought is that the tractor had gotten so it would only start if it was towed nearly half a mile. Turns out the valves need to be adjusted. Who knew? Not me.:)
There's not much to go wrong with these systems, but a few things often get overlooked.
