I have been doing a bit of searching for transmission choices for this K, and would like your opinions if possible.
My initial thoughts were that the simplest choices would be:
1. Use the transmission section from another set of donor cases as Tim did (LHS clutch, RHS drive)
2. Use a Harley big twin transmission (LHS clutch, LHS drive, although there may be RHS drive options available?)
3. Use something like a BSA A10, or even ES2/16H style box (LHS clutch, RHS drive).
Without too much thought at that point, and having briefly seen them in the past, I wondered if a cassette-type box from a Japanese sport 250 might not be usable. Similar hp rating, cheap, dry clutch, 5 or 6 speeds I think?, and compact, lightweight quality with further race ratios available should the bug keep biting.
I hunted down some pictures of a late 80s box:
If those don't work, here's the link: http://www.flickr.com/photos/21629196@N03/
After an initial inspection and much mental gymnastics it appears to my untrained and inexperienced eye that this is not feasible. It has a RHS clutch and LHS drive arrangement that I can't see would work unless the whole things was flipped over.
One can see the Norton box used here in Stellan Egeland's impressive "Hustler" bike: http://www.seservice.se/mainpage.htm
Can anyone suggest any other interesting options?