The FX-51 core is the same as the Opteron 148 , with the exception that the Opt 148 is validated for full ECC operation, and the FX-51 is not. (The FX-51 can do ECC, but it isn't 'validated' to do so.
I believe the MP capabilities in the FX and the Opt 1xx series is cut in the actual die itself, and there is no way to enable it using the external pins.
Jeremy