1/35th scale Bell 47, I'm assuming you mean either the MRC (47D-1) or the Revell (47G-2) kits (Tamiya doesn't make a 47).
Building the boom from brass is possible, a modeler in town has done it and claimed he used 12 feet of rod. I haven't read the article, nor have I constructed my own boom. Instead I was happy with the slightly oversized boom supplied by MRC.
As far as aftermarket accessories go I don't know of any conversion or update sets. The 47 was/is a simple machine so most of the extras you may want to add can be scratchbuilt. Do a search of the web, there are several good sites with 'walkaround' photos (including www.aircraftresourcecenter.com).
Hope this helps,
Trevor