O.K. here goes. He is now liable to you under the law for "like kind and quality". you can argue that you did not have rebuilt motors in the boat so you need new ones. He could argue that a freshly rebuilt motor has less wear than an 80 hour new motor. It could all end up in court and you'll have to pay me huge bucks to come and argue your side as an expert witness
Or......... there is a possibility he could get this handled by his garagekeeper's or business liability insurance. There would be a deductible and his rates may go up but there is an upside for both of you.
He can get paid to do all the work. He pulls down the engines, and finds all the damage done, you supply receipts for the previous work showing new motors, he makes up an estmate for new motors(like kind and quality), and both of you end up o.k. Most companies will pay normal customer pay rates for mechanical work so his profit from the job may cover his increase in insurance and deductible. I have seen it happen many times.
Feel free to email if I can be of any help.