Welcome to the Fiero Community! Here will you find (literally) more than a decade of knowledge, experience, and hotly defended personal opinions!
Besides the mall here, places you should keep in mind for parts are fierostore.com and of course good ol' Rockauto.com . Since you're working on other cars, it's likely you already knew about the latter.
Some things are getting tougher to find, like rear brake calipers. As suggested, take the time to check out your frame first, if you have the room, patience, and skills, taking all the body panels off and checking everything out would be good. Including pulling out the carpet!
Also, yeah, pushing the limits of the stock Fiero engine with power adders is not a popular idea. There is a book out there - "High Performance Fieros" written by Robert "Greg" Wagoner. It's worth checking out, has a lot of solid information in it. Also MANY enviable build threads on here. Some people put in excellent details, down to the part numbers of things they used, or where they sourced them from.