I wouldn't steal anything from them! Just cancel the check and tell them to just take you to court for the money, cause if it's like you're telling it then no judge will rule on there side. But from a "legal" side you first have to tell them your not going to pay and you want your bike back. If then say pay or no bike, then pay with a check and then stop payment. If you do ANYTHING illegal like take something your screwed in court! I come from a family of lawyers and thats what there advise was. And they said it's also a possaility that the court will rule on there side so make sure you get as much of the bills and written estimates they should of gave you before you stiff them. That was you have a case in court.
Hope that helps and every thing works out for you!
Jeremy