No chance??? Don't you just love those throw away comments. A stock internals Gen III will do 400hp happily (mine did) and with a very short life would pull 500hp. All 500hp requires is forgies and the necessary bolts on that any other 500hp engine would require. I have a number of mates and know of quite a few 3SGTEs with 500hp at the fly and over....some pushing 500+hp at the wheels. Takes a well thought out build for sure bit nothing any competent engine shop couldn't produce and for far far less than 30K
If you want no chance then it's the CT26 pulling that hp. You might get 350hp flywheel from a CT26/CT20B combo if you know what you're doing but even a heavily modded CT26 just can't flow those numbers thru the turbnine housing no matter what you do to it.
What GEN engine isn't the only thing i'd be worried about if i were you..you need to find out a whole lot else about what you have bought as what i've heard so far dosen't sound right at all