Do I have to replace flywheel, PP and disc or just the OEM disc?
I don't wanna be spending a lot of money on the whole kit if I can just replace the OEM disc.

 

Just the disc; though I would highly recommend changing the PP and resurfacing/replacing the flywheel at the same time since you'd be paying practically the same labor. You'd be saving money in the long run by doing it all at once.