It is currently a civil matter. Now as that car sits on your property if he decides to pick it up and move it, then it matters who’s name it is in. By your story it sounds like it is in your name. If he moves the car report it stolen if you so choose, but don’t obfuscate any facts or your knowledge of who took it or why because then it’s a false report. Also know that when you do that he’s going to go to jail for felony take motor vehicle without permission. The reality is he is never going to pay you and you probably get to keep his 700$ but in that world he’s probably also going to retaliate, remember when you described him as a ‘sketchy ex junkie’? Yeah, those guys tend to hold grudges. Decisions decisions

If you ever want idiots just being idiots for the sake of seeing who is dumber, often times praising themselves, go to a YouTube comment section