Jibran, in my opinion that if you were driving normally and car has no existing conditions i.e. white smoke, excessive oil consumption etc. than this case should not be related to tappet or cylinder. Sudden loss of power is not caused by wornout piston rather it can be related to ignition, fuel injection or timing (which is rare in 1KR since it has a chain)
Just check your coils, coil connectors, Injectors and injectors connectors. weirdly enough the injectors and coils in 1KR are located in a zone prone to excessive heat and vibrations.
easiest way is to start the car and start disconnecting coils one by one. Then coil that doesnt have any effect on engine idling upon disconnection is the faulty one. repeat the same with injectors. That should help you isolate the problem.