1996 Mazda 626 running rough

Ok I have a 1996 mazda 626, it has a 2.0L and about 234000 on the car. I was driving to virginia from florida when the car broke down in south carolina. A mazda dealership told me I had a crack in my block.(Ripped me off!) So they went and put a junk yard tranny in it. I have got a year on the engine now and the other day the light came on, and it started to stumble when I got on the gas. I had the computer scaned and they said it was a o2 sensor in bank one is that the top or bottom one? 2nd question is that I changed the spark plugs out then I checked the wires which are not but a year old and only have 4000 miles on them. They where good?( I think) at least whith the car running I pulled one off at a time and it made a difference each time I pulled one off.L what if there is something else wrong with it that is causing the o2 lcode to pop on? It was said to be running rich? What can cause this. The cat had been emptyed out. Could that have something to do with it but it was done after the light came on. I really dont want to get rid of the car because I like it and I have about $5000 in it. The tires all need to be replaced and the 4 wheel alignment needs to be done. New used moter, new belts, new battery. And also I have tryed 2 different o2 sensors out of a junk yard(cheaper than paying $300 for one new one) and it still does the same thing. Please any and all ides will be assepted. Thanks guys for all the help!