Artrain,
The engine in a QG 4000 is an E095H.
First, it is splash lube engine and if it has made metal, it is toast....
Second, contact local small engine shops. Onan engines have been used lots of places and a number of manufactures have been making engines that are nearly "bolt-ins" because Onan prices are so high.
Third, it may be substantially less expensive to merely replace the engine part of the set. The engine and generator are actually different parts that have few connections.
Forth and final, Please pursue the suite against CW. It may be problematic, because if you put any hours on the engine since it was there, they can try to make the case that your lack of attention was the cause, but there is also the good chance that someone there drained the lube oil and neglected to refill it.
Matt