You cannot install airbags in a Fiero. First there isn't any room in the steering column and none of the later airbag equipped vehicles has a steering coulmn that will fit a Fiero. So you would need a custom made steering column. Second, the wiring, sensors and controller would need to be integrated into the chassis. You would need an automotive engineer to review the chassis in order to properly place the sensors and the airbags.
By the time you did all of the above, You could have bought a brand new car with airbags and saved a lot of money.