If it is 12 volt negative ground, it was modified somewhere along the line. This tractor originally was 6 volt, positive ground.
As for sparks - this is a sign of a poor ground connection. Completely remove the cable at this connection. Clean the paint and corrosion down to bare shiny metal. Now clean the terminal on your cable. Reconnect everything, using a good clean bolt. After all this, paint or otherwise weatherproof this connection.
Now when you crank the tractor, you might notice a day and night difference in how fast the motor spins. BTW, this is a very common electrical problem. Many overlook it when troubleshooting - they aren't given the tell-tale sparks to bring this to their attention.