The S&W 916's were never a well reputed pump gun when made 50 years ago, nor very desirable today. I checked Numrich Arms/ Gun Parts Corp lists a 916-A 12 gauge trigger plate assembly as in stock and available... for $225.
So while this part and the firing pin are available, in my opinion their replacement cost exceeds the value of the whole gun. I suspect that is why you were given the gun for free.
For the money you'd have to spend to fix your free 916, you could buy a used Mossberg 500 or other more reputable pump and have a way more functional pump gun than the 916.
That may not been the answer you were hoping for but is my pragmatic view. Welcome to the forum and good luck.