SSEI engine replacement

1995 Bonneville SSEI - engine appears to have locked up and it looks like an either a rebuilt or used engine will be needed. Is it possible to replace the supercharged engine with a non supercharged, or would the necessary changes be too extensive?

It isn't impossible but why would you want to. The supercharged engine is far superior to the standard 3.8 in performance. If you buy a used engine the price would be about the same without needing to purchase the associated components like the PCM. It would make life a whole lot simpler to stay with what you have!