What you can do is a full size drawing of the corner, and lay out your own pattern that falls within the scale of the existing trim or will replace it. If you search the net under headings like "crossette", or "Greek Ear" or just subjects like "exterior window and door trim", or "architectural trim", you may find examples, and details. I've done similar designs on cabinets and entry doors, but they were my own pattern. It does get tricky when the profile has to follow and then have a transition point. It's almost like one of those "mazes", if you follow what I mean.
So, doing a full size drawing of the detail, or a scale that is large enough to really see, like 1 1/2" = 1'-0", or maybe 3" = 1'-0". If you don't have an architectural scale ruler, pick one up, even one of those cheapo white plastic triangular ones.