The at fault insurance company (the one who rear-ended you) should pay "full replacement value".
That doesn't necessarily mean paying for an off the rack brand new bike. But, to pay to either build up a similar bicycle from similar components, or buy an equivalent used bike (or pay you the value so you can acquire what you wish).
If your bike is > 20 years old, you might have this question moved to either Classic & Vintage (where someone undoubtedly has dealt with insurance valuation of an older bicycle), or to Advocacy-Safety (periodic insurance questions).
https://www.bikeforums.net/classic-vintage
or
https://www.bikeforums.net/advocacy-safety/
To get a topic moved, either click on the red button
, or ask someone else in the thread to do so if it doesn't work.
The insurance company may try to low-ball you. But you don't have to take their first offer. Agents may not understand differences in bicycles.