There is not an engine builder alive, that hasn't at one time or another, let something slip through the cracks........... So let them fix it for free and move on, or don't, then take a huge loss parting it out, or PAY someone else to fix it. Seems to be your choices. I think I would try the FREE one first.

Stuff happens. I built a motor for a member on this board. Took me WAY too long to get it done, but when it was, I ran it on the dyno and it make good power, was sealed up good, made good vacumn, no blowby. He gets it in the car, makes a few passes and it is oiling a plug....BAD. Checks the usual things, so we narrow it to the rings. I send him some rings for that hole, puts it back together, no change. Now by this time, he is pretty darned upset with me. I offered to have it shipped down at my cost and have a look, but he elects to have somebody else look at it. I tell him, if it is something I did wrong, we will TALK about the bill, but I won't guarantee anything...........Well turns out the head developed a crack where it was ported so thin, by another VERY well known and VERY respected Mopar guy on a previous build and was oiling the cylinder. So as I said, ANYTHING can happen to ANYBODY.