The mechanic I hired to install my engine didn't install it properly and the bolts are coming out?

So my sisters husband and his dad are "mechanics" this is what they do for a living. About a year ago I hired them to install a new engine in my car. They did the job. My car sat because I lost my job and I couldn't register it. Anyway now that it's legal to drive my husbands coworkers took a look at it and found out that all the bolts are loose, or just put for show. After they did the job a year ago they wanted more money because it took them more time than expected. They said they needed to install the a/c band and that they would finish installing the band once we have them 300$ more. That was insane but I said no. We paid what they charged and that was that. When my husband found out that the job was a Mickey Mouse job. My sisters husbands answer was "you owe me money" and my husband said "you said the car was good to go" my sisters husband just laughed. We live in Los Angeles. What can we do. We have a 5 month old baby that was in te car as well. We are taking it to Honda to get it evaluated. What can we do legally? They are street mechanics and I'm sure not the first victims of their scams. We thought we could trust them

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