Also some rifles are just stuborn. Pour some dish soap down the barrel and then fill it with boiling water. Sometimes if people use a lot of bore butter it ends up filling the fire channel. If the water comes through the nipple fine, then dry it and blow air from a compressor through the nipple port.

It might be something as simple as he used old caps, bad powder or not the right powder. It would be interesting to know what kind of powder he was shooting out of that .54. I have the same rifle and it shoot fine. I use Goex Black Powder or Graf's & Sons black powder. Try some black powder in it, with a good cap if the bolster is clear.

Thumper is right about what the rear screw is for. It is for a tang site or base plate to mount their scope.

As for the other part, if you can't make one then you have to buy it. I would have guessed Dixie Gun works had them.