You could ask the bankruptcy court to exclude the bike from the bankruptcy on a theory called "resulting trust," which means that your stepfather was holding the bike in trust for you, and the formal requirements of the trust were never created, therefore the trust fails, and the bike "results" back to you.
That's the "ivory tower" solution. The problem with it is that unless the bike is very valuable, the cost of your hiring a lawyer to represent you and plead this theory, may cost more than the bike.
Another alternative would be for you to enter into a new installment sales contract with your stepfather now, for a price aproximately 70% of fair market value. Then you would make payments based on the contract, and when he files bankruptcy, those payments would belong to the bankruptcy trustee.
In effect you would be buying the bike a second time. But, since vehicles generally go down in value over time, your debt may be a lot more reasonable now, than it would have been originally.
This may seem silly, but the cost of a lawyer will probably be more than repurchasing the bike.
Or, you can just let the bike go and move on.
Hope this helps.
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