Do I really need a top end engine rebuild?

First had trouble when EPC light came on. Car became sluggish and started misfiring on 2 cylinders and was occasionally chucking black smoke from exhaust. Took it to the garage and they said it’s probably ignition coils and spark plugs. I’ve had these replaced and it isn’t misfiring anymore however it is still overfuelling. After further investigation by the garage they now think it is a top end engine issue that is causing this however I’m more inclined to think it could be something electrical because when the light initially came on, throttle response became extremely slow/ non existent and I know this could be because of the 2 misfiring cylinders but felt more like the car was struggling to sense the throttle was being pressed. Mainly looking for advice on wether to get the top end inspected and worked on or if it could be something else entirely