Thanks for all the replies. We really like this diesel, it's hard to find so we intend to keep it.
A special thanks to @MartyE for showing us the replacement part that we could not locate.
I decided to super glue the broken piece together so the engine can at least be used as a pusher.
Once the replacement part comes, we are going to see if the super glue will hold the weight of a train.
Assuming it doesn't, I will switch out the broken piece with the replacement, once I've painted it to match.