Most people will simply tell you to get an attorney for this.
I'm not a fan of those people, but sometimes it is the right move.
I always handle my own insurance claims because I am persistent and competent, and apparently have a weird fetish for pain.
If you can't be bothered to deal with the insurance company yourself, then you need to find someone to represent you.
This being said, an attorney won't be interested if there isn't at least moderate personal injury.
An attorney takes their fee from the insurance payout for pain and suffering, not usually property damage.
Onto the aftermarket parts:
I had an identical situation with a motorcycle I had. A very nice, older bike that had a market value far above its "book value". Additionally, it was nicely modified with aftermarket exhaust, excellent Corbin seat, racing suspension, upgraded brakes, etc. In this situation, the bike was totaled.
The insurance company initially offered me $750 due to 'book value'.
I declined their generous offer.
This particular adjustor, as with many others, was not rude or aggressive, but simply said "
that's all I can do until you provide me with documentation that shows the property damage is much higher".
So I hunted down dealership and ebay listings (only the actual sold ones matter) for the bike, and then tallied up MSRP for the aftermarket items.
I eventually added up the extras to ~$3k. Which is somewhat significant for an older bike.
I believe we settled on a compromise of like 50% (can't remember) of the MSRP of the items. In this situation, 100% value wasn't appropriate.
So I got ~50% of the $3,000, plus $1500 for the bike.
I did not provide receipts to the insurance company as I bought the bike modified, and never had the receipts. I just told them what the item was, and sent them an email with the weblinks.
Obviously, custom work is very hard to be reimbursed for. Although the bike had very time and cost intensive DIY work that added excellent value and utility, it just turned into a "if you can't put a dollar amount on it...
"
So there ya go.
Be polite, but firm.
I always verbally expressed to the adjustor "I know your job is hard. I know you have a boss to report to. I get it. I'd like to make this as easy as possible for you. Here's every single document you need to justify to your boss what I'm asking for."