Repairing my Sealand Toilet was an easy deal. I broke the spring cartridge but decided since I was in there I would just replace everything in the ten year old toilet. Depending on age you may want to do the same. But, yes, the spring cartridge and the water valve are the most common culprits. Parts are common and easy to find.
I do all my own stunts
03 Dolphin LX 6355, Workhorse W22, 8.1 vortec, 04 CR-V, Blue Ox, Brake Pro----Norm, Barb and
Doc(He's a PhD)