If your boyfriend owes you money and won't pay, you can sue him in small claims court.

If you have items that are in your possession and were unconditional gifts to you, they are yours. If he wants to claim that they're his, he can sue you for their value or try to obtain their return through the courts.

If you tell the police, "I'm holding the items until he pays me money he owes to me," you can expect that you'll completely undermine your claim that the items are yours and, at best, have the officer instruct you that you have no right to hold his property hostage.

You're not going to be able to pull parts out of his car.