My standard advice is to fit a Disklok (from Halfords). Big, clunky and inconvenient, but exceptionally so for a thief, and beautifully made. I'd guess that your car can be so easily identified that it will have been broken up for parts, or had its identity changed with a rusted or damaged example of the same model. So check Autotrader, etc., and classic car auction lists for any similar age MGB roadsters coming up for sale.