Not a good situation.
If the title is in your ex. name, but the loan is in your name, there is not much you can do. If you were going to finance the loan, you should have had your name put on the title.
If the title is in your ex. name, he is the legal owner of the bike, no matter who's name the financing is in. If you take matters into your own hands and take the bike, this could be considered theft and you would be the person in trouble.
You may have more success contacting the place that financed the bike. They will have a lien on the title. They would have the legal right to repossess the bike. So they may be able to help.
What may help is that you have a signed agreement. I would contact a lawyer and take him to court. You may also be able to take him to small claims court and avoid the lawyer fees, however the dollar amounts involved may be too large for small claims court.