I believe you'll find the model availability is fairly limited. Valterra and Thetford seem to be the main players in this market; etrailer.com and RVupgrades.com both have several different types.
There doesn't seem to be a universal size. There is a downside to installing a replacement that's a little smaller than the hole in the ceiling that you can set up a situation where you will be forcing air into the ceiling cavity rather than out through the vent. Obviously if you choose one that's a little larger, you will have to cut the hole in the ceiling out a bit. This is a great opportunity to check and make sure the cut outs are taped up good; there are a number of videos showing how to do.
I bought and tested several different styles measuring airflow with an anemometer before deciding on my final choice. I ended up using the Valterra A10-3359VP everywhere except the bathroom where I used a Valterra A10-3354VP. The 3354 is an old school spin up/down to adjust but it will open a lot and is a 7" diameter. I found that the bathroom being so small doesn't really need too much air and it's location along the racetrack duct is in an area of higher pressure. I keep mine turned down quite a bit and if you should need more air, it's very easy to reach and adjust.
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