The standardized o ring part # is 123. The pads are much more difficult. I have made them by reducing the size of standard norton ones in a bench grinder. Used ones, prefarably, as the Dunstall pads are thinner. To figure the thickness needed, measure the distance between the pistons fully retracted, subtract disc thickness, divide by two, and that is how thick the pad needs to be (minus a few thousanths). Rummaging around for used ones might get you in the ballpark. To get the diameter, centrally locate the old Dunstall pad on the "new" one, spray it with contrasting paint, and take it to the bench grinder. Wear a dust mask. Yeah, it is pain in the ass. The second time I needed pads, I bought a Norvil disc kit from Phil Radford. Option # 2. contact Vintage Brake here in California, as I think he can make them. If he can make them, chances are someone closer to home can too.
Couldn't put his link directly, but his web address is
www.vintagebrake.com/
Another possibility is to contact Phil Radford at Fair Spares America. As I recall, he had a customer who was re-producing Dunstall pads ( or having them re-produced). If Phil doesn't remember, he is getting on, I'll jiggle his cobwebs for you. Good Luck!