All buying and selling is done in the 'Mall' section of the forum. You should have no trouble locating all the parts you need. Also, spend some time with the SEARCH feature, and you will find that there is NO question about Fieros that has not been asked before, and you can read all the responses.
Seems kind of absurd to go to all the work to put it back to stock. You have the makings of a fun car. Archie made a great kit and it sounds like it is mostly installed with the fab work done. You can get a custom harness from the mall and cpu recommendations from the forum. If you want a stock Fiero - find one, buy it and have 2 great cars.
IS there a specific vendor that sells the harnesses fort the L99 tailored to the Fiero?
Engine harnesses are made by a few forum members, usualley the 3800 SCII engine, so thats a posibility for the harness for your engine. If you plan on useing the stock motor, I would swap out the whole cradle.
Was the Lt1 supposed to be a 1970s LT1 OR , a mid 90s LT1 ? Is the L99 the infamous 262 chevy small block from the mid 80s ? ;-( Use that kit and cradle and get a Different Chevy Small block.(for the weight of a Fiero, seems a 302 would be a good choice." Or if you WANT the "Electronics" a newer LT1