The issue may come down to precedent, repair cost and risk aversion. Insurance (and real estate) rely on "comparables" for price estimates and taking on risk. Hard to find 818 and GTM comparables.
You will likely find similar circumstances for a hand built original. Uniqueness and volume are factors. A discontinued car makes it less attractive to insurance companies. They may look at custom fabrication and no service/part support as justifying the car as a likely write-off in any claim. They establish rates or reject coverage as the car being a worse case scenario when damaged. I do not know, but I suspect the majority of typical insurance policy claims are not "totals".
jim