Your worst case situation is the insurer paying out market value minus what they would get for it from a salvage company and allowing you to keep the car. Being able to show receipts for recent work to improve the car would increase the likely payout to an extent. Tracking is just maintenance.
At the end of the day, you do need to be realistic that it is a 19 year old car with a market value around its scrap value. Discussing the offer with the insurer's valuer/engineer politely as opposed being on the offensive charged with emotion would be my recommendation. The engineer will not be out to insult you!
These things happen. It's not the end of the world - as I kept reminding myself when first my pride and joy Rover 820 and later Rover 420 were written off many years ago.
