How do I fix this missing soffit?
As mentioned by other comments and posts, the soffit needs to remain vented. For what it is worth, that one piece won't matter if it is perforated or not, (the whole soffit is open on the inside, that is a good thing) but use perforated soffit anyway so it matches. This material is available at any big box store.

The picture have captions in them that should explain what I seen needing to be done. What I have illustrated is the simplest way to proved a repair that will last without taking the fascia and soffit down. The only detail I left out was how to fasten the soffit panel in. Once the block is in that I drew in, add a piece of "F" track that will hold the soffit piece at the wall, or it can be screwed directly to the block. The outside egde can be screwed in too but the added fascia "extension lip" to replace the one that was torn off, is made to hold that end of the soffit in place.