Did you already register the motor in your name? If so I don't see how you have any leg to stand on when it comes to getting the pistol back.
The original deal appears to be done and now you have a warranty dispute. I'd either make contact with him and work it out or just go pick up my boat and make sure everyone knows the guy isn't a straight shooter.
This. If that warranty was written and you can document it, you could possibly file a lien. Consult an attorney or your county bar association for a referral.