Gotta agree with JohnRR, the best approach is to park your emotions at the door. Be as calm as possible, say things like, "We've got a problem and I wondering what you are going to do to make it right."

Also, and this is important, know what you want and can reasonably get as a remedy. You for sure need new pistons, as some have cracked oil ring lands by your description.
You need to find out how the engine was assembled. Were the rods gone through with new rod bolts? Was it balanced? Stuff like that. Know some ballpark prices for parts, like you'd find in a 440Source catalog.
And keep the personal invective out of it. He (and you) may well know that he's a lyin' piece of s@#$ but you don't need to tell him. That won't get the job any closer to done.

I know how hard this is, I haven't been the best at following my own advice. But it seems that when I do, it turns out better.

R.