I've got series 1 127 which I believe has the smaller early tripod (nb g/box end) joints. If I wanted to replace the gearbox with a later 4 speed with the bigger tripods could I just replace the tripod joints on my existing driveshafts? (The joints are still listed as available new).

Failing that I guess I could use the diff off the old gearbox, but is that a realistic option without professional help?

Inner tripods were all the same on the 903cc gearbox....later 1050/1300 were bigger.
903cc starter motor is on the other side to 1050/1300 ohc units so make sure you get ohv (903cc) gearbox

Membership Number: 10521 yes that is a membership number!