I think it depends on insurer and car. I've had mine insured as a classic for a while now, I think they do allow for cars with classic or cult potential. I would be surprised if a 1991 would not make it into the classic bracket.

In fact I would think that for escort cossie owners it would be worth asking the question now.

I could not get classic insurance for an 83 motorbike but I could get it for my 88 saph so it's always worth asking.