Eddie
First off , nice gun .
Now for the bad news go to the FAQ in the H&R center fire rifle forum and you will see that a rifle barrel will not be fitted to a shotgun frame by H&R also it is not a safe thing to do yourself .
You will find some info there as far as replacement parts like springs , hammers , ect.
Depending on the age of the gun some or none of this info may be helpful .