How to tell if my car will be written off?

Goudy

Have you tried to source the screen from a JDM specialist here in the UK?

Have you tried the JDM forums?
You probably aren't the first to crack a screen on one of these, whatever it it.

I had some trouble with a rather limited edition motorbike that was only available in small numbers and mainly only in Japan.
A local importer helped with getting hold of some used parts that even sources and dealers in Japan were telling me were unobtainable.

The good ones don't just import mint vehicles for resale, but often fill up shipments with other, less than perfect examples to be broken as parts.

Also, you might not need to go all the way to Japan for parts.
There are other markets that are flooded with JDM cars and car parts nearer to home.

Nebbit

After this long delay I can report that the correct windscreen has now been sourced by Autoglass and fitted under the insurance. This involved a lot of negotiation that would not have been possible without an insurance company with a UK call centre. 

not2plan

Well done.
Popped in to say similar.

JDM import, very specific colour to the glass different to UK model vehicle.
Autoglass sourced the windscreen from Japan plus OEM seal to fit it at about 4x the cost of the UK equivalent.

Totally justifying why they make you pay more for grey import insurance and happy to take advantage of that.