Email my friend Martin Struk at
rsenthalpy@gmail.com and he will set it right. Trust me on this. PM me if you want more details, but Martin is the guy to go to if you are exhausted with trying to get the original shop to sort things out satisfactorily.
He might be able to help you track down an engine too -- feel free to PM me or do a little googling to find out more about him, but he is the #1 person in FL (or really anywhere) for Nissan rebuild, tune, etc.