Looks really quite good Ivo.
The core can only be handled with a seperate set of short exposures (about a minute?). You compose a completely seperate image out of these (don't worry about the outer noise that will be present) and then blend it into the middle regions in PS. Takes a little fiddling and trial and error. The thing to watch though is that the transition between the core area and the other middle nebula regions isn't obvious, this is the hard bit to get right...and very few Orions get it right in my opinion.
Have a go
Mike