You need to weigh up what it’d cost you to replace it.
Lets say it’s a 15k car to replace, plus and mods you have, wheels, suspension, engine build receipts, etc.
Leys say just as an example your car value and mods list stands at 25k.
Id get an agreed value of 25k on the car. Tell them that’s what you want.
I do that with Adrian Flux.
When I wrote off my saff a few years back they paid out the agreed value, no issues.
Cheers Paul