The only parts you as the form 1 builder can replace without paying another $200 tax are wipes (rubber disks with slots in them). You can send your silencer to an FFL/SOT class 2 for repair. The only part that can't be replaced by an SOT is the tube. The FFL/SOT also cannot make repairs that result in a longer tube or a change in the bore.
As far as I know another person can make parts for you as long as you are present and control the parts; as in take them home at the end of the day.
Here is the BATFE silencer FAQ; http://www.atf.gov/firearms/faq/nati...silencers.html