I am brand new here, but I think my Monster started out as a 750 and the guy told me it has a 900 motor in it. It looks like a plug and play thing from what he described it to be. I still need to verify if my Monster is a 900 motor because it doesn't seem as fast as my 900SS with 39 FCRs/Lighten Flywheel/Exhaust. There is a MAJOR power difference when I rode both of mine so I'm skeptical about my Monster right now.
Well, you've also got aftermarket fueling, exhaust and lighter flywheel- which can make a lot of difference.
But like DP said - if it's got a dry clutch, then it's probably a 900. If it's wet, then odds are it's still 750