Decals
Bob Jackson's may have some appropriate old transfer decals for you, if you choose to repaint the bike with them they will look into it. I asked for some JRJs recently, but they won't sell them on their own.
I have found some JRJ decals from Nick at H Lloyd cycles, but they are more like a 50s ish design, at a guess. I happened to be looking for Merlin too as I knew the frame I am repainting may have been badged either way (no original decals left), unfortunately for you Nick has none.
I have seen a couple of stills of a Merlin (poss 60s) on Classic Rendezvous, with similar but not the same designs on, inc. good photo of an old head crest with a large M and a bird (a 'merlin' perhaps?). Also I saw one hanging up in a classic/old bike shop in North London called Sargeant's.