Modified car valuation - Woodsport Celica V6

[C_Hemingway]

I am looking for a professional valuation for my vehicle with regards to a non-fault claim
My vehicle is a category N write-off with a market value of £500

I have declared the following modifications with my insurer:

• Non-standard engine - Toyota Camry 3.0l 24V V6 ( Woodsport conversion, 3VZ-FE ) - Approximately £5000 at the time including travel costs
• GT4 ST205 turbo bodykit - Front bumper, bonnet and rear spoiler - Approximately £500
• Original racing colours ( 'SEGA Rally' Castrol WRC livery ) - £1500 at the time

The people who should be on my side are challenging me to find evidence of it's true value

Otherwise they are trying to force market value as professionally modified Celicas do not generally come up for sale

I am not looking to sell and am only interested in a valuation
I will be prepared to pay for this service with this in mind

Any help or advice would be greatly appreciated