Apply for a title to an abandoned motor vehicle at your county tax office. If the previous owner has reported it stolen, this will show up and you are not liable. They can come get it and you must let them. If it is not, then you may apply for title.
If the previous owner does not want the car and you have applied for title, (it may have been too much trouble for him/her) you may get a clean title to it after a period of (I think) two years and a provisional temporary title.
In any event, by doing so you are not in violation of the law, which you would be driving a potentially stolen car.
Is there a mechanic's lien against it?
Paul