Wow!
Thats a very nice bike, I'd love to see it in person. It's cool that it runs the same little gearbox. I've seen quite a few of the boxes for sale in a fairly short period of time. For as many years as the boxes were used, you'd think we could find the secret stash somewhere, but despite my best efforts, I can't find it.
The good news is that these boxes are avail, so it's only a matter of time. Although mine were complete, I wanted to run a fresh clutch. Turns out, you can just rebuild them, all you need is a box of corks. Funny to think of cork as a friction material, but from what I read, it was very common.
I'll do some more digging, if I can find a clutch, I'll let you know.