My go to offhand and hunting rifle had a 42", .45 caliber, 15/16" barrel when I was a young buck. Now that I am old and decrepit, I just can't hold it steady anymore. It is really your choice, but it is on the heavy side. I suggest you try it before you buy.
Regarding the lock, if it really is a Siler percussion lock and not the Chambers mountain lock (which would be more appropriate for a mountain rifle), I believe the Siler flint lock is a drop in replacement and you have to do is remove the drum and install a liner like Rich said.
Dale H