Yes, there are a few problems, but it has been done. The swing arm location is different on the Comet when compared to the twin. It is an inch forward and a half inch up, if I remember correctly. Also, the shortened wheelbase can cause the rear wheel to fowl the crosswise fender brace on the rear down tubes of the Egli twin chassis. Thus, on the Egli Comet frames, this brace had a deeper curve in it than the twin. All other dimensions are the same. These issues can be overcome with some ingenuity and welding, although I have never personally seen the modifications on a machine. If you have an original Egli or Slater frame, these modifications would probably diminish the value of the frame. However, if you do not have a frame you can easily purchase an Egli Comet frame from Andy Sidlow or Cyril Malem. It might also be worthwhile to sell the twin frame and buy the correct Comet frame.
The Norton box is a good idea and has been done on many Comets. I make my own plates and I have chosen a slightly different location for the swing arm, but there is no magic, it is just a bit of work making plates. If you are going to attempt this type of project, I would suggest that you join the VOC and correspond with the Egli Section Organizer. He can be surprisingly useful with technical details and photographs.
David