When you think about the electrical system of your Case, the only items that draw current that are not attached to the engine block are the two headlights.
The item that draws the heaviest current is the starter and that's why it is important to ground the battery to the block. That said, I agree that having a modest grounding cable between the block and the frame is a good idea to ensure that headlights and the voltage regulator are properly grounded.
You said that you replaced "both" cables and that implies "two", not three. You have a cable to ground the battery, a cable to supply power to the solenoid and a cable to connect the solenoid to the starter.
Keep in mind that Kohler's are fitted with ACR or automatic compression release. This feature opens the exhaust valve slightly to allow the starter to spin the engine over easily and when it reaches a certain RPM, the valve closes. If you do not have your valve lash set properly, then the starter could be fighting compression and struggling to get the engine spinning.
You can also do a test to see if there are any problems with your electrical system.
Remove the spark plug wire so that the engine will not start. Then spin the engine over using the key switch to get a sense as to how fast the engine spins.
Then use a set of decent booster cables to connect the negative side of the battery in your car/truck to the engine block and then connect the positive side of that battery to the big lug on the starter solenoid that hosts the wire going to the starter. The starter motor will instantly begin to spin. The issue is....does it spin faster than it did before?
If it does spin faster, then you still have a problem with the electrical system in the tractor. Recent battery or not, perform the tests I suggested. The hydrometer test will show if one cell is weak and that's all it takes to make the engine spin slow.