I researched getting one for about 4 years. I always wanted one, but decided to get new cars instead. So I know quite a bit about them for never owning one.
You can put just about any GM engine into a Fiero, and probably all of them have been done before...LS1, LT1, 3.4L OHV, 3.4L DOHC, Quad 4, 3.8L, 3.8L S/C, 4.3L, 3.8L Turbo from a GN, Northstar V8, etc, etc.
The S/C 3.8L is the common "easy" swap and there are a lot of parts out for that engine. I actually really like the 3.4L DOHC swap. It's a unique motor, can rev freely, has a different sound, etc.
P.S. - SpaceGhost...did you used to live on the east side, off of 161? That pic of your Fiero reminds me of the one that used to sit across from my buddy's place.