Aerodonamic did a replacement in my 86SE when it was property of a previous owner. He got the replacement tray from The Fiero Store. He said was a big of a booger to replace, but I would bet that it's much easier while you have the engine out of the car. Mine is held in place in some areas by stainless steel screws. I can see those when I remove the battery, but I don't know how other anchor points are secured. Contact Aerodonamic to get his take on the replacement.
BTW, tradition is that when you have the engine out, you must take a picture of "Standing In The Engine Bay" and post it on the forum.