Pro here, lots of questionable advice on this thread. I have a lot of experience with old plaster. This could be repaired with a piece of drywall but that’s not the best way to do it. It’ll look different, especially over time. To do a proper plaster repair I would first coat all edges of the existing plaster with plaster bonding agent. This seals and stabilizes what’s there and keeps the old plaster from wicking the moisture out of the new plaster - which would make a weak bond = which will cause a crack over time. This is an important step even if you were to go the drywall route. I’d also coat the lath with bonding agent. This will help the plaster stick instead of just relying on the keys that are formed when the wet plaster squeezes through the spaces in the lath.

When that has dried the product you need for the first coat of plaster is called Structo-lite. It’s a plaster product that is made with perlite, that keeps the weight down and gives it some texture that helps the next coat to bond to it. The Structo-lite gets troweled over the lath pushing pretty hard to try to form new keys as it squeezes through. Sometimes a nice straight board will help to get a flat surface - use it as a screed to get a consistent thickness. This is just the first coat so it doesn’t have to be perfect. When this dries it’s rough texture is perfect to bond the next layer. Durabond joint compound mixed from powder is the thing to use but be careful - it cannot be sanded so don’t leave high spots. The final coat will be a sandable layer - either Easysand mixed from powder or regular premixed joint compound. This layer can be sanded when dry to get a perfectly flat wall.

The Structo-lite when applied with the plaster bonding agent will expand and contract with seasonal changes in temperature and humidity the same as the old plaster and will not crack at the joint with the old plaster like many of the other techniques here will.

I’ve been restoring old houses for 35 years and the old timers who taught me this stuff are long gone.

https://www.usg.com/content/usgcom/en/products/walls/drywall/plasters/structo-lite-basecoat-plaster.html

https://www.usg.com/content/usgcom/en/products/walls/drywall/plasters/plaster-bonder.html