This past winter while rehabbing my 1975 18SF I replaced that brass tube with some pre-fab 1" ID FRP tubing from mcmaster-carr.
I was re-doing my transom and bought the tubing to replace the brass scupper drains.
Figured since I was already making dust I might as well replace the fish box drain as well.
Drilled out the old brass tube with a hole saw (hole-saw inside a hole-saw trick) and a forstner bit, and with a little grinding, some epoxy and some glass scraps I now have a nice watertight drain for the fish box.
Was pretty easy.
Pre-fab glass tubing - 5ft stick (way more than you'll ever need) less than $20.:
http://www.mcmaster.com/#8535K26