Hi Mark,
I ended up buying an electrical (9volt battery) one. It is a 2.75 litre foam extinguisher. I managed to fit it upside down in the passanger footwell. A 3.5litre would have been too large. The two nozzles supplied point towards the carb/exhaust manifold. I managed to fit the firing button and control box inside the cabin. Have a look at Merlin Motorsport.
http://www.merlinmotorsport.co.uk/
Paul