I think I would have taken everything off the car that I had bought for it, and given back the remainder to them.
Do they have the right to seize your property, if it was placed on the car after you purchased it????
Especially if you have receipts for all the modifications on it then it is not part of the original purchase and therefore was not sold to you by the previous owner, so therefore (imho) should not have been seized because it was not part of the original loan agreement.
The original loan covered the cost of the car as it was not as it is now.
Get a good lawyer and try and get some of the parts that "you" paid for, not the loan company, back.
Best of luck and I hope it works out for you.