I purchased my 3rd int154 that had a slipping clutch. I inspected the clutch disc and found it to be still thick in size. I was using the tractor to level my driveway and found it wasn't slipping when i used the creeper even with the tranny in 3rd gear. After giving it a good hard workout I noticed the clutch is much better now. I think it only needed to be reburnished in again. It must have been glazed really bad to have been slipping. I did purchase a new clutch disc and pressure plate already so down the road i'll change it out. But for now i'll get some more life out of it. I got lucky for now. Don't baby your clutch, deglaze it once in a while. You may want to try it before you replace the clutch. Put the brakes on and let the clutch up.
My dad years ago would put the car against a tree and deglaze the clutch in.
On my 4x4 standard tranny truck i would check the clutch in low/low in 4 wheel drive. I would hold the disc brakes on and let the clutch up. If it would drive thru the disc brakes the clutch was ok. If it kept running and not drive thru the brakes the clutch needed replacement. My first clutch from when the truck was brand new lasted for 120k miles.