there is a few ways to going about this.... first have them total it and get paid for the modifications by presenting reciepts ( if his insurace company allows this), get paid for the damage if the car isnt totalled. Buy the car back after they total it while getting a salvage title on the car or which Ive done in the past a few times ask the most they will pay "you" before totalling the car or putting a salvage title on the car.... and use the money that was paid to fix and mod out your car more without worrying that you decreased the vaue of the car with the salvage title
(Sorry if this didnt make total sence cuz Im drugged out on vicoden right now weeeeeeeeeee)