Halon only really works in confined spaces where it is toxic to humans. Military training films show the extinguishers being let off and dropped into turret hatches and the lids closed behind so the gas can form a blanket.
I tried to fight a Carburettor Fire once on a Land Rover and it just fanned the flames.
Co2 is probably best but forget about finding ones to fit the brackets. Leave the others for show and get a big Co2 Extinguisher.
Depending on what period you are portraying with your Saracen, Halon might not even be historically correct.