https://www.monacocoach.com/service-and-repairs/
The earliest is a 2000 model year. I lists the 350 HP as standard but below that it shows 500 ISM as an option so maybe in 1999 it could have a 450
I'd suggest getting the coaches SN, should be on a tag at the front door and then sending a request to the above link and get a copy of the MSRP sheet and the data sheet which will list the engine.
Or if you can get the engine SN go to
https://quickserve.cummins.com/info/index.html and create and account and then put the SN it, you can look up the engine info there.
There is a data plate on the engine, pulley end passenger side but sometimes hard to see.