If you already signed the title over to him, it is legally his. If you put a lien on it, then it is officially yours until he pays in full. I doubt you went through that though.
You could be a really nice guy and deliver it to his house with a note explaining that he can get you the 500 bucks whenever he can afford to, and that someone should be enjoying the bike, and since he is the legal owner, it should be him.
Or
You could part it out, and if it is contested that you sold property that was not yours, then produce any documentation showing that he owed you 12,000, but didn't pay in full, then you tried to contact him for the rest and to get him to take the property, but he never did. I'd consult a lawyer on this first though. A lawyer might be a few hundered, but if the bike was worth 12k two years ago, I'd think you could part it out for at least 4k now.
If you need to find a current address for him, zabasearch.com works pretty well.