This is a classic case of one person's (yours) against another's (the seller), and the only way forward is to marshal the facts.
The company offered you a free replacement motor, and your approach should be that you want them to honour the promise they made. The fact that the discovery of the cut cable was made after the offer was made (and accepted by you) mitigates in your favour.
Be firm - offer made, and offer accepted. What happens while the machine is in their hands is hardly your problem.