Do you know what your rear dropout spacing is? It might be too old for 140 mm. If you have 140, you might be able to find a used wheel with threads on the hub for an Arai drag brake. These brakes are not being made any more and appear now and again on Ebay. If it can be accomidated, it is the best alternative. There are also some early Phil Wood drum brake hubs around, but these were difficult to keep adjusted.
If I was faced with many miles of 10%+ descents, I would probably opt not to use this bike!