Unless your dad or you know for sure (have notification) that the tractor has a legal lien (either by bank or it's listed in a bankruptcy equipment list as an asset) then your dad still owns the tractor. Buy the tractor from your dad for a fair price and get a bill of sale from him. Document the condition of the tractor - photos etc. before moving it or fixing it. Then it's yours and you've acted in good faith.
You can have your dad hold the money in trust if you're still concerned.