I'm guessing you have a Japanese Miroku Bess with 42" barrel.
They came new with the flattened trigger guard. The Japanese received a musket that received a ding in transit and assumed that's how they were suppposed to be. They never had sling swivels.
That is the highest quality Brown Bess ever made...you have a very quality piece there.
I absolutely love the Japanese cherry wood!
I've chosen to leave mine alone, no holes drilled for sling swivels etc.
You can buy repro swivels but not any kind of replacement for the trigger guard that will just install. That would come under the heading of skilled work, and who could or would do it?
The front sight could probably be replaced by a gunsmith, here's what they look like. (save the pics)
There are repro bayonets specifically for the Japanese Bess, the others won't fit right. But bayonets will scratch up the metal finish.