There are a number of engine selection you can put in a Fiero.... heck I have seen someone put in a Ford 5.0 EFI! The selections are numerous so I'll tell you why I decided on  GM's 3800 Supercharged V6.

Update Summer 2003

Well as I moved on to other aspects of the build we took out the Diablo to have some work done on the suspension and when we loaded it on the trailer all power to the transmission went out... I feared a blown transmission because no matter what gear it was in it did not move an inch.  Well we had a few choices, but to make a long story short, we decided to scrub the 3800 and sell it to go with a more authentic, more powerful longitudinally mounted V8.  After a month of planning we decided to install a 2000 Corvette LS1