Adbarrett,
Smitty is tellin' you the deal with the vaccum leaks. They could be just about anywhere there is an old rubber or plastic hose -with some cracks, pin holes, or frayed ends. You just need to slowly check everything over with a careful eye. Replace anything that looks shadey.
No, you cant use the o2 sensor from your 83 on a 90. FCP Groton (fcpgroton.com) is an internet import store, w/ excellent prices and good service, that will give you the best price on whatever you may need.
You still, however, can not over look other variables. When was the last complete tune up (plugs/cap & rotor/wires/ fuel, oil, & air filters) done on the car? My suggestion would be to buy a copy of a Haynes or Bently service manual and use it to stay informed (ie. before or after you see a mechanic) or to help you fix the car yourself. There are alot of things that can lead to poor economy and you need to eliminate each one by its "heaftyness" on the wallet. Peace, Jon