The first problem that you are going to run into is that the codes you are getting are OBD1 (probably) codes, not OBDII which Toyota phased in during the 1995 model year. All cars sold in the US since the 1996 model year use OBD2 – which means there are few cars on the road today that use older systems. That means that many mechanics are going to have no idea how to read or interpret codes for your car.
As far as I can tell 21 is probably an Oxygen sensor fault and 52 is indeed a knock sensor. I don’t know what 02 is. Might be a modifier for code 21 identifying which O2 sensor? Anyway, here’s a link to a list of codes for a 1993 Lexus which is about the closest I could come to a 1994 Corolla with Google in a reasonable amount of time. http://us.lexusownersclub.com/forums/index.php?s=0d7b220696d5922a134962c7eb188d01&showtopic=26394&pid=347747&st=0&#entry347747
As for what’s wrong. I haven’t a clue. Maybe somebody around here with more rapport with engines than I have can make some useful suggestions. If I had to make a guess, I’d think maybe timing, but you didn’t mention engine roughness or lack of power so very likely not timing.