You should be able to get a valuation done by a recognised Cosworth specialst garage. Not sure in your area who you go to to get work done on the car but they should be able to help, plus submit lots of photo's and you will be able to get an agreed value that is realistic. As cars like your Escort Cosworth and my 2wd Sapphire get older and parts are getting harder to find ,accident damage if the worst was to happen is getting more and more expensive and if it was stolen and not recovered it is an even worse situation. For instance if your car is rust free and in top condition trying to find one as good could be really difficult. Bear all this in mind when you come up with the valuation.