Anders,
My Mk1 has been fitted with a gearbox, just the usual procedure re leadscrew etc.
Also replaced the old clutch with a Mk2, straightforward job if the complete assembly is used, the parts of the Mk1 and Mk2 are not interchangeable.
As John says, the belt guard needs modification, I chose to "lift" mine by using blocks under the mountings, and reversing the spring clips, This just avoids extending the cutouts at the side. A hole was needed in the drive belt cover to accommodate the end of the clutch shaft.
Edited By Robbo on 13/04/2017 08:59:52