Hmmm, the ID of the BB-shell is 38mm or 1.50". I don't think we can thread it for any commonly available replacement bottom-brackets. Threadless BBs won't work either as they need the 33mm ID shell to press out against.

The only thing I can think of is to get a replacement English-threaded BB-shell from a framebuilder. Spin the OD down to 38mm or just over for a tight interference-fit. Then apply a thin layer of strong 2-part epoxy and press it into the current BB-shell. Now you can use any off-the-shelf replacement bottom-bracket.

Here's an item that might fit just right. You'll just need to trim off 1.5mm from each end to be 70mm wide:


http://www.cycle-frames.com/bicycle-...-LIGHT-BB.html

The OD at 38.1mm may be a little too tight to press in, so milling it down to 38.05mm should be about perfect.