Thanks for the comments guys.
SR2 Magneto - good kit if the SR1 is anything to go by, get it rebuilt if needed and get the auto advance and retard unit done at the same time.
17" wheels - Scottie, you will be welcome to the rusty rims in due course! 18" WM3 for the rear and 19" WM2 for the front is what I have in mind, Morad TC alloy or stainless steel for preference. I'm sure your local wheel builders can find you a set of 17" rims in much better condition, but tire choice for these is less impressive than for 18" rims, though not impossible. Hopefully losing 1" of ground clearance wouldn't cause you problems.
And yes, I have already spoken to B.W. about this, it is highly likely I shall be asking him to build whatever engine it ends up with - as Asbo # 12 has the fully sorted AVL lump, I'm thinking of something more traditional (looking).
Regards,
A.