Indeed which is why I have replaced both the old halon job and the power job in my engine room with a halon replacement. Gael Force were doing good deals earlier in the year on them.
Sea fire 200 from Ocean Safety give them length, depth, beam of engine room and they will tell you the apropriate size. should not use haylon now as dangerous when working in engine room. New ones are fog gas.
For your engine space, 1 x 3kg, (halon replacement) type auto extinguisher, fitted so it will totally empty, job done.
FM2000 or the like, not dry powder!