You can strip off the B and see what you can turn up. The A, B, and C grades refer to the hfe of the transistor. A is the lowest (100 I believe), B is the middle grade (250), and C is high hfe (400). The numbers are free from memory, so probably inaccurate, but you can check the data sheet to make sure.
There's usually no harm in using a BC546C instead of a BC546B. Also, if they're used in the protection circuit, you can probably use any NPN/PNP of similar specs that fit the pinout. Even something like BC327/BC337 would probably work.
Tom