The bolster clean out screw is not worth $25 buck to have replaced. Many of the new T/C rifles do not even have one. If you do a good water bath, that will clean the bolster just fine.

Get some JB Bore Paste and give the barrel a good scrub. It will not remove the pits but it will take the rust and any grime out of the bore.

Pyrodex will work just fine in that rifle. About 80 grains and a roundball will shoot just fine. Just make sure you give it a good water bath after ward.