Tony,
You should not have much problem sourcing bits for the postwar Mark series. Between club spares for the wear items, autojumbles and adverts in the NCR (and this forum of course!) for the non-reproduced bits, you should be able to piece it all back together. The biggest stumbling block seems to be good cylinders (they are awkward to re-sleeve) and the special 274 Amal carburetor bodies if the latter are missing or broken. It will not be as easy as doing up a Triumph of BSA, but all the more rewarding when you have accomplished it.
As you encounter problems, break them down into individual, or at least related topics. Do not run twenty problems into one post as the thread of responses can get a bit messy.
Best of luck with it,
-Doug