I agree with Ron 100% about the difficulty factor, and other things mentioned.
I started my P11 hot rod in the early 70's and I couldn't find OEM parts or any documentation related to the P11 back then. I was young and the internet wasn't happening. Impatient and didn't look hard enough with my phone book. And I have never been a club guy, so out of the loop with the guys in the know. It is easier now to search and find information, but parts unique to the P11 are getting rare as far as I can tell.
AMC Classic Motorcycle Spares still use the original part numbers in the White Master Parts List 1966-19667-1968 that are shared with the Matchless and P11, and they carry parts nobody else seems to want to stock. Also very reasonable pricing compared to popular US Norton parts houses. Having the parts in stock in the US does not make them worth a 100% mark up. And on top of that they overcharge for shipping in the US. Sad really
By the way, the brakes, hubs, and primary cases parts that are wrong for the P11 in the illustration examples in that ebay book are illustrations out of the Norton Twins section. So to be expected.
Speaking of puzzling P11 uniqueness. I made the mistake of getting ready to replace my rear tire and thinking I should replace the rear hub bearings while I'm at it. What a fluster cluck design. I looked for days to find the parts for that hub. AMC ended up being the only source I could find for those parts. It's a good thing a few Matchless diehards are still around.
Enjoy the P11 experience, and sharpen your search skills, cuz you're gonna need them.
If I am repeating myself, it's because that second C-word 19 shot gave me a darn headache.