I'm no lawyer but if both of you are on title both of you have equal rights to the vehicle and components. He couldn't just strip the whole thing and leave you with 50% of the shell. Same premise and I'm sure a judge or arbitrator would agree. That being said - it is easy to spend way too much on lawyers so I would recommend you come up with an amicable agreement and try to get it executed. If he wants to keep the motor give him the car as well and insist he compensate you for 50% of the value of the car and improvements. Then go find another...you probably don't want the car around anymore anyway if it reminds you of him.