@Daniele
Yous keyboard adaptor should be fine to be able to let you get into the BPPC menu. Mine works (Power Tower). The PPC eats lots of power, but IT IS possible to make it run with the normal A1200 PSU... try to take your machine apart, hold the mobo somewhere and attach the bppc, then just connect the A1200 PSu to the mobo, connect the composite video of the Amiga 1200 to a monitor and turn the computer on, see if that works, it's not possible to run the bppc with this PSU all the time but at least it should start... I've done it several times.
Good luck,
D.
EDIT: Fixed typos