You should perform a leakdown test to be safe. I can't believe it was sold to you in that condition -- that's very shady. Previous owner obviously knew this was a problem... the bolt or nut HAD to go somewhere. It was likely removed.
You should replace the entire turbocharger. It's trashed.
Forgive my previous conclusion. 99% of the time someone says "my car won't hit target boost" it's not the compressor wheel. You are a very rare exception.