I went down this road recently while insuring The Black Bike. Because many of the parts are custom* I have to document all of my work and show what was put on the bike, and attempt to document the replacement cost in terms of material and labor.
I got some good advice involving bolt-on modifications. If you've purchased parts for the bike, keep all the receipts. Keep photographs of the parts and accessories on the bike. When it comes time to make the claim, you won't have a leg to stand on unless you can prove that these parts were on the bike.
Fred
*custom: (n) A part which was designed and built without mass production, to fit a specific application, and which cannot be ordered from a catalog.