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Whether you are liable depends on a couple of factors.
What was the condition of the truck when you got it? Did you make it worse? When it was brought to you, could it have been fixed entirely if it had been taken to dealer at that time?
Ok, by the time it got to dealer, I still need to know if it was in worse shape.
You are not responsible for the cost of a new motor, in any case. When they brought it to you, it was not new... at most you are responinsible for any additional damage you did, if any.