Toilet cistern

The cistern takes a very long time to fill up and it's very loud - I'm timing it, it is now 17 minutes and counting. We removed the lid and checked inside, the whole thing looks a bit worn down. We filled up a bottle of water and put in the tank to help it fill up faster but it didn't help much. Water slowly trickles into the toilet during the fill-up process. Toilet seat also needs to be changed but because the toilet is built into the wall. Not sure where the seat bolts are as they are not under the toilet. (We just moved in here) any idea if it can be fixed? Or is it better to replace the whole thing? Can send pictures for assessment but not sure if that's possible on this website.