From the parts list (
Resources - Bristol Cars - Owners and Enthusiasts Forum) it is a Smiths FT 5301/17. I believe that it is essentially the same as other FT5301 units apart from the arm. I think your arm should basically be 9 1/2 inches long with a slight kink. The FT5301 was used on various other cars including early MGBs and some Midgets so you could get one of those and fashion a new arm. If yiu can't get them from the usual sources, there is an American company that lists units for these cars on eBay who will supply a unit with a longer arm if required.
I don't know what the difference is between FT5301 units and FT5300 units which look similar.
Have you checked the old sender unit? One reason for failure is when they get seized from resin and oxidation in the petrol tank - you might be able to fix yours. You might also find that you have a rather grubby petrol tank.