Don't worry, I too thought it would just be as easy as getting a new gear set but doing a bit more research into it burst that bubble..
I like what they have done with those Hewland gearboxes, a splined layshaft makes ratio changes really easy, it seems all of the local gear cutters reuse the std layshaft and weld new gears onto them.
I want to know the shift pattern for those boxes, do they still have the std (or dogleg) pattern or a bastardised pattern to make 5th 1:1?