Since a new clutch is now $500.00, a good used clutch for $125.00 is a bargain. Just make sure he is selling you one for a Kohler and not an Onan.
You are going to have to learn clutch repair and clutch adjustment if you wish to continue owning your Case. On this site we have a Technical Library. In that library you will find Service Manuals, including several for the clutches.
Here is a link to take you there.
viewtopic.php?f=25&t=629
You should begin by unhooking the wire to your headlamps.
Then, remove the hood hinge pin so that you can remove the hood.
There are bolts that hold the oil cooler mounting brackets to the frame. Remove those and the cooler will be able to swing to the left side once you deal with any muffler shrouding.
You now should have a clear shot at the clutch. READ the clutch manual several times before beginning to dismantle the clutch.
CONSULT the manual every time you remove another item and lay those item out in ORDER of removal on a clean work bench.
You are now trying to find out which items in the clutch are defective.
According to you, the disc is OK so what failed? Was it one of the three bearings? If so, then replace all three NOW. These are available at any bearing wholesaler in your area.
This is not a super difficult job if you take your time and read the manual carefully as you progress with the task. Inspecting each part is important and especially the condition of the main shaft that everything is mounted on. The more grooved and worn it is, the harder it will be to get a good adjustment. Sorry but there is a learning curve here that you must master on your own. There are no magic answers here that will solve your problem. Learning about the clutch comes with ownership but we are here to guide you along the way if you are confused. A digital camera will help us assess any parts of concern.
Even if you buy that used clutch, you will still have to dismantle it totally to install it on your engine and then put it back together correctly prior to adjusting it.