I would check to see if a lien has been recorded against the tractor. How this would have been done would depend on your state, but in many states the lender will file a UCC lien with the county clerk. You can find more information by googling "ucc lien" and the name of your state. You'll want to check for liens in the name of any family members who might have used the tractor for collateral, not just your father and grandfather.
What you don't want to do is to sink several grand into the tractor, just to have the bank come and get it once it's running. Banks "write off" bad debts, but they don't write off liens. If a lien is recorded against the tractor, you must get it released before you spend one nickel on the beast. It's quite possible that, knowing the condition of the tractor, the bank would be willing to release the lien for a nominal payment, but that's totally up to the bank.
Now, if there's no lien recorded against the tractor, then possession is 90 percent of the law. Maybe even 100 percent. With no recorded lien, the bank has no more right to the tractor than someone driving past it on the road.
You don't say if your father ever declared bankruptcy, or if he intends to do so. If he filed bankruptcy, then the tractor should have been part of his assets to be disposed of by the court. If he declares bankruptcy in the near future, giving the tractor to you (or the bank) could be considered concealing assets, and the court could come after the tractor.
IANAL, so this advice is of course worth exactly what you paid. Good luck.