A lot of respected writers and composers have adapted traditional material. Burns, Grainger, Copeland and Vaughan Williams come to mind. Mind you, they had all demonstrated very clearly that they were capable of writing their own material as well, and were not simply depending on making small, "fashionable" changes to stuff they didn't have the imagination to make up for themselves. Demonstrate that you can do as well, and you'll be well on the way, though even the best interpreters of tradition today often have difficulty finding a publisher. Of course, there is quite a market at present for cobbled-together "cross-over" music, so it's probably not terribly hard to get some backing in that area if you put the work in. If you genuinely love the music and treat your sources with respect (and acknowledge them, having done your research properly), instead of just using them as a handy royalty-free resource -as all too many people do- then you will deserve to succeed, and I'll wish you well.