How can I tidy up a hole in a plaster ceiling rose?
I don't know what the right answer is, but I'd probably go with attaching an adhesive screen, using non-shrinking plaster/spackle over that to fill in the area, then trying to tool the surface to vaguely resemble the rest of the design. If you were trying for a museum-quality repair the answer would probably be to actually take a mold of the opposite side and use that to replicate the leaf pattern, but I couldn't advise you on how.

Fixing butchered art is never easy.