try contacting these guys - I've always found them to be pretty reasonable to deal with:

or try some of the other international dealers:

if he only broke the knob, and the shaft is fine, then he should be able to find another rewind knob that will fit...I would try a newish Cosina or Vivitar or Ricoh SLR (plastic body one) - I don't know for sure, but I wouldn't be surprised if the SLR part fits the Bessa (sorry - it's been years since I've seen a Bessa)