I replaced mine with a $30 unit form Walmart. It's a little smaller than the hole. I used sheet metal screws to attach it to the old mounting plate. There's a bit of a gap around the edge but I feel that's good as it allows for ventilation. I've been using like this for several years.
By the way, there was a recall on my Samsung microwave. I called them with the model number and after a lot of back and forth with a local repair tech who had to verify something Samsung sent me a check for $200. It might be worth you checking into.