I would, at least, try to get the money back. At most you will have a claim for what you're spending now.
I own two automotive service shops. I am very reluctant to get attorneys involved in auto repair, but have been forced to a few times.
There is a company called Donan Engineering that does forensic testing of parts to determine cause of failure. If it came to it you could use them to prove your case in court, that the repairs to your engine were crap.
I know this sounds like overkill, but I'm guessing you're out thousands of dollars because of these con men.