I just want to thank Bob Salomon for referring me to Mike Boylan at Blazes Photo in Toronto. Blazes has the necessary parts - knob and shaft assembly, plus casing, both new - and can do the repair/replacement. That's good news. Thanks especially to Mike Boylan for responding so quickly. Mike took my call immediately and sent me an e-mail, within a few hours of receiving a photo from me showing the problem, confirming that he can fix this, and at a cost that I consider reasonable.