dont put any heavy oils in that engine and check the breathers and PCV for clogging, clean them out anyway. Also check valve clerances and adjust as required, Put good engine oil (something like a light semi synthetic oil), fill up some fuel and take it on a hard run, hard means drive like you just stole the car or drive it like its a sarkari car.
the cat will also get cleaned and so will the whole exhaust.
before condemning the piston rings for wear make sure to do a compression test.