Gibson are certainly an option, but why do you assume that their quality is better than the overseas models? Gibson have certainly turned out their share of rubbish over the years, and Mastertones are no exception. The low-end Mastertones like the RB250 are overpriced, and there are better banjos from Deering and Stelling in the same price bracket. There are lots of high quality banjos from smaller makers, both in the US and overseas. Most of the small luthiers will build a replacement neck for your current banjo, which is generally regarded as a good solid bluegrass instrument.
Try here for some advice and sources for necks.
Regarding 'overseas' banjos; look at Gold Tone, and Lebeda. I think Jiri Lebeda (from the Czech Republic, 'George' to his friends) used to do a lot of work for Stelling.
Hope this helps.
Tim