Sorry Eddie51 doesnt actually work like that, i agree in what you are saying but in reality it very rarely happens. Unless it is a major mod i.e.bodykits/ serious alloy wheels,spoilers etc.the ins. co wouldnt be aware and certainly there engineers wouldnt pick up on an engine mod, again unless it obvious ie air cleaners etc.
. Most accident work is done by imaging and more often than not the engineer doesnt get to see the vehicle.If by any chance the repairer spotted something they would keep quiet about it, they need the work !!
Obviously if it was a serious or fatal accident and a police examiner was involved then thats a bit different.